On the rain-swept planet of Gallifrey, a weary President Romana is greeted by an annoyingly cheerful Doctor. He’s overjoyed to see that K9 hasn’t aged a bit and hasn’t got a speck of rust on him, and he also comments on the scale and grandeur of her rooms -- dark and mysterious, Gallifrey at its most typical. Romana compliments him on his new body, but although she’s delighted to see him she’s curious to know why he breached the Transduction Barriers and materialised his TARDIS right outside the Presidential Palace. He tells her that he needs her and K9 to accompany him back to Cambridge on Earth in 1979. They were there once before, in the old days, and he reminds her of the time they punted up the Cam only to be abruptly taken out of time, kept in stasis for a while, then put back again. They’d intended to visit Professor Chronotis, an associate of the Doctor’s who resides at St Cedd’s College -- but neither he nor Romana can remember why they were there. K9 recalls that the two of them were gone for nearly two and a half hours and when they returned the Doctor announced it was time he made some changes as they were going to Brighton instead. What’s odd is that the Doctor had a dream about Chronotis last night, in which he saw his old friend calling to him. Perhaps he still needs them to do whatever it was he wanted them to do in the first place? Romana points out that the Doctor could easily go without her, but he doesn’t want her to be plagued by the mystery and he suspects that they’ll all be needed if it’s going to be solved properly. He pleads with them to go back with him... back to Cambridge, 1979.

In deep space, the Institute for Advanced Scientific Studies is under quarantine and a recorded message warns ships in the area not to approach. Professor Caldera contacts Skagra, demanding to know what’s going on, but the strange scientist merely tells him that the others are waiting for him in the main laboratory. Skagra explains that he's called everyone together to tell them about the emergency, but when Caldera arrives he’s shocked to see their fellow scientists are all completely motionless and attached to a hexagonal unit. Skagra claims that they’ve had some sort of accident and that he was hoping Caldera could help. They’re not physically dead, but Skagra doesn’t think they’re ever going to be quite right in the head again. He offers to show Caldera the end result of Think Tank’s work and proudly produces a large white sphere. Skagra explains that the scientists’ minds are now contained inside the sphere... and Caldera’s is about to join them! Caldera screams out in agony as the sphere attaches itself to his head.

In Cambridge, the post-graduate student Chris Parsons arrives at St Cedd’s College and asks for directions. Moments later, he knocks on the door of Professor Chronotis’s rooms and finds the elderly lecturer making tea in an adjoining room. Chris introduces himself and apologises for intruding on his time, but the Professor says that at his age time doesn’t matter too much, not that he expects Chris ever will get to his age. He recalls talking to the last Master of the College but one (or it may have been three) -- a nice young chap who died rather tragically after being run over by a coach and pair -- but then he forgets what they talked about as it was such a long time ago. Chris reminds the Professor that they met at a faculty party a few weeks ago, and he was kind enough to say he would lend Chris some books on carbon dating. Chronotis returns to the kitchen area and tells Chris to help himself, but his directions are rather garbled and because Chris is late for a seminar, he collects several different volumes at random. As he’s preparing to leave, Chris asks Chronotis about the old police box in the room, but the Professor doesn’t know anything about it and assumes someone left it there while he was out. It fits quite nicely in the corner though.

The Doctor and Romana make their way through the streets of Cambridge, guessing that the reason K9 never accompanied them on their last visit was because he couldn’t get over the cobbles. It’s the middle of the afternoon and the note on Professor Chronotis’s door said he’d be back by tea-time. Since then, the Doctor has taken Romana on a tour of four colleges, six second-hand bookshops and an art gallery, but with any luck the Professor will have returned by now and put the kettle on. Romana thinks it’s odd that he’d arrange an appointment with them and then go out, but the Doctor thinks that sounds exactly like Chronotis as it would be terribly unfashionable to have tea ready before they got there. The Doctor is in the middle of recalling a charming evening he spent with Chronotis on Lake Geneva with Lord Byron and Mary Shelley, when Romana tells him she can hear voices...

Chris returns to his college laboratory and calls out to his colleague Clare Keightley, telling her he’s got the book he needed from Chronotis. There’s no reply, but he thinks that’s typical for Clare as she’d be late for her own funeral. He examines the first book and realises straight away that it’s not the one he wanted -- and that it’s written in a strange language.

The Doctor and Romana finally arrive at St Cedd’s College, Cambridge, which the Doctor proudly declares was founded in the year something or other, by someone whose name he’s forgotten, in honour of someone who for the moment escapes him completely. St Cedd, probably. Suddenly the Doctor spots a face he recognises and goes over to greet the college porter, Wilkin. He’s surprised when the porter remembers him, but then he did take an honorary degree here in 1960. Wilkin also recalls that he previously visited Professor Chronotis in 1964, 1960 and 1955, and the Doctor points out he was also here in 1958, although he was in a different body then. He and Romana bid goodbye to Wilkin, knock on Chronotis’s door and are welcomed in. The Doctor instructs Romana to sit down and correctly predicts that their host will ask them if they want tea, then ask if they want milk, whether they want one lump or two, and then finally whether they want sugar. The old man emerges from the kitchen and greets his friend the Doctor warmly. He introduces Romana, and Chronotis is sure he’ll have heard so much about her soon. He explains that when Time Lords get to his age, they tend to get their tenses muddled up. After offering biscuits, he then suggests crackers, to which the Doctor agrees “sometimes”.

Wilkin is engaged in a telephone conversation with another prospective visitor to the College when he’s approached by Skagra, the strange scientist dressed in flamboyant futuristic clothing. He rudely addresses Wilkin as “little man” and demands to see Chronotis! The porter explains that the Professor won’t want to be disturbed as he’s with the Doctor, a very old friend. Skagra informs him he will return later and leaves.

Chronotis tells the Doctor and Romana that he’s lived here in the same set of rooms for 300 years, ever since he retired from Gallifrey. One of the delights of the older Cambridge colleges is that everyone is so discreet that no one’s noticed he’s been here so long. He asks what he can do for them, but the Doctor points out that they came here at his request after he sent them a telepathic message. Chronotis denies sending any message, but then he has an idea who might have sent it -- himself... but it was so long ago he's forgotten why. He assumes the Doctor must have a better idea what it was about, since he heard it more recently, and the Doctor wonders if it has anything to do with the babbling inhuman voices Romana heard earlier. Chronotis thinks that’s probably just the under-graduates talking to each other, which he’s been trying to get banned, but they think it sounded more like ghosts screaming. The Professor suddenly remembers why he asked them here -- it’s a rather delicate matter regarding a book...

Chris telephones Clare and tells her to stop being busy as he’s got something important to tell her. If she wants to see the world of science turned on its head, she should come to his lab right away. He tells her he’s got the most amazing thing -- a book with a molecular structure unlike anything he’s ever seen. He thinks it might be extra-terrestrial, and he’s run all the obvious tests including x-rays and spectrographs, but now he wants her to see it for herself.

Skagra is nearly knocked over by a car as he walks down the middle of a quiet country road. The car screeches to a halt and the driver gets out to accost him, then realises the man is acting strangely and offers him a lift. Skagra accepts and manhandles the man back into his own vehicle, telling him he needs his help. He asks the driver if he knows how to operate the car, and as it turns out he’s actually treasurer of the Ford Prefect Society and knows the vehicle backwards. Skagra explains that he needs to get to a field a few miles away and the driver assumes he must have broken down there. Without warning, Skagra produces the large white sphere and it floats towards the driver and attaches itself to his head. The man screams in agony... and moments later, the car drives off with Skagra at the wheel.

The three Time Lords in Chronotis’s rooms are certain they just heard voices, very faintly this time, and the Doctor wonders if it has anything to do with the book Chronotis mentioned. He assures them it’s just a book he accidentally brought back with him from Gallifrey, although he then admits it wasn’t an accident and he overlooked the fact that he deliberately decided to bring it. Romana is horrified that he kept such things for study and the Doctor agrees they could be terribly dangerous in the wrong hands. Chronotis agrees, and now that he’s getting old, he was hoping they might take the book back to Gallifrey for him. The Doctor and Romana start working their way through the enormous collection of titles on Chronotis’s shelves, and stumble upon a huge text covering the five principles laid down during the great days of Rassilon, but this is just a Gallifreyan nursery book which Romana read as a Time Tot. Chronotis tells them this isn’t the one he meant and admits to bringing back the odd two or seven books, but there’s only one that was in any way dangerous. It’s called The Worshipful and Ancient Law of Gallifrey, a little red book about five inches by seven. The Doctor is outraged that such a book was removed from the Panopticon Archives but Chronotis argues it was safer with him (in theory). The book dates back to the days of Rassilon and is one of the Artefacts. They have no idea what might be in the book, as Rassilon had secrets and powers that the Time Lords of today don’t understand, but at least there isn’t much chance that anyone else will understand it either. As they resume their search of the shelves more urgently, Chronotis recalls that the book might actually be green...

The books in Chronotis’s study continue to pile up... Roget’s Thesaurus, The British Book of Bird Life in Colour, Alternative Betelgeuse, The Time Machine, Wuthering Heights, Little Grey Rabbit’s Birthday, Tandoori Chicken for Starters and Sweeney Todd... but there’s no sign of The Worshipful and Ancient Law of Gallifrey. The Doctor is starting to wonder whether it’s all that important anyway apart from its historical value, but Romana reminds him that each of the Artefacts was imbued with stupendous power, and although the meanings may be lost now, the powers and the rituals remain. As President of the High Council, Romana has no choice but to report Chronotis’s actions, but the Doctor thinks a wise President would be able to see the bigger picture. He argues that leaving a powerful Time Lord relic lying around on a Class 3 planet like Earth is really only a little crime, and in any case the High Council wouldn’t know how to react. Romana believes this is nonsense and reminds him of what happened to Salyavin. True, he was a great criminal, but he was also a boyhood hero of the Doctor's as he had great style, flair and panache. The Doctor never met Salyavin as he was imprisoned centuries ago, even before the Doctor was born, but oddly neither he nor Romana can recall where. Realising Chronotis was a contemporary of Salyavin they ask him if he remembers, but he changes the subject, having remembered the student who came here earlier to borrow some books. Chronotis struggles to recall his name and starts working his way through the alphabet A... B... C...

Clare has joined Chris in his laboratory and agrees that the results he’s shown her are incredible. They’ve used a top of the line electron microscope and discovered the book doesn’t seem to have any molecular structure, as if it’s made of pure non-atomic matter. This is clearly impossible and she thinks he may have rigged the equipment for a joke. Chris denies this and points out that the book also seems to be absorbing radiation from the air around it, even when there isn’t any to absorb. He suggests they conduct another experiment on a more practical level...

Chronotis has nearly reached the end of the alphabet when the letter Y finally prompts him to remember young Chris Parsons -- born in 1957 and graduated in 1978 with an Honours Degree in Chemistry and currently engaged in post-graduate studies on sigma particles. He tells the Doctor that Chris is likely to be in the physics lab in St Simeon’s College and gives him directions. The Doctor leaves, promising to be back in 15 minutes, but whispers to Romana that if he’s not back in two hours she’s to lock herself and Chronotis inside the TARDIS and send out an all-frequency alert. After he’s gone, Chronotis starts to wonder if he gave the Doctor accurate directions, but it’s too late now anyway. He offers Romana another cup of tea.

Chris asks Clare to try tearing one of the pages from the book, but she can’t. She can’t even cut it with scissors. She thinks it’s probably made of some kind of plastic, but Chris has already checked and there’s not a single polymer in sight, nor is there any crystalline structure. Half of it seems to be stable all the time, but the other half isn’t stable any of the time. It also behaves like a super-conductor one minute, then blows up his equipment the next. There’s no way of telling what it’s made of, so Clare suggests trying a spectrographic analysis, but again he’d already tried that and it caused the machine to blow up. She asks him what the book’s actually about but he doesn’t know as it reads like a cross between Chinese and algebra. She suggests the obvious thing to do is ask Professor Chronotis and she’s surprised that he hadn’t thought of that.

The Doctor approaches Wilkin outside the college, gets clearer directions to St Simeon’s and notes that it’s almost exactly where Chronotis said it wasn’t. After he leaves, the porter is accosted again by Skagra, who is this time wearing clothes stolen from the driver he killed earlier. Wilkin confirms that the Professor is now alone, although he’s not sure he should let the visitor go up without an invitation. Skagra pushes him aside in frustration and heads for Chronotis’s rooms. He examines the sphere and tells it that it will soon feed on a mind of quality...

Disaster strikes when Chronotis finally runs out of milk. He’s still feeling angry with himself for losing the book, but Romana assures him they’ll find it eventually. He notices she’s shivering and she admits to feeling unnerved by the voices. He goes to make her a cup of tea, then remembers he has no milk. He offers to go and buy some more, but Romana doesn’t think that’s a good idea and she goes to fetch some from the TARDIS instead. He recognises the ship as a Type 40, which first came out when he was a boy, and says one of the main complaints about it was that the kitchens were an intolerable distance from the control room. After Romana goes inside the ship, Chronotis is left to consider the fate of Salyavin, who he dismisses with the words “good riddance”. There’s a knock at the door and he invites his new visitor in for a cup of lemon tea. Skagra enters carrying the sphere, and the babble of so many inhuman voices causes concern for the Professor as he only has seven cups. He’s therefore surprised to be confronted by Skagra, who demands he hand over the book he took from the Panopticon Archives. Chronotis tries to bluff ignorance, but Skagra threatens to "deduct" the information from his mind if he doesn’t co-operate willingly. He sets the sphere loose on Chronotis and warns him not to fight against it or he will die.

The Doctor arrives at Chris Parsons’ laboratory, but Clare says he’s just missed him and offers to take a message. The Doctor spots what he’s looking for on the table and when Clare tells him it’s an odd book, he says its owner is an odd fellow too. He asks her if she’s read the book and when she says she couldn’t, he wonders if she means she can’t read. She’s curious about the origins of the book as the writing looks like an explosion in a spaghetti tree, and when she tells him it blew up the spectrograph, he decides to take a closer look at the equipment.

Inside the TARDIS Romana invites K9 to join her outside as it’s turned out not to be just a routine social visit after all, but first she asks him to check how old the milk from the kitchen is. He tells her that it’s been in the stasis preserver for 32 years and is still perfectly fresh. They return to Chronotis’s study and are shocked to find the old man apparently dead on the floor. There’s another knock at the door, and Chris enters and sees Romana crouching over the body. K9 reports that the Professor is still alive, although he’s in a deep coma. Chris explains that he came to enquire about a book that Chronotis lent to him, and she realises he’s the young man they’ve been looking for. When he says he just came from his lab, she asks why the Doctor didn’t come back with him and it becomes clear that they missed each other. K9 analyses the Professor, and discovers that he’s been subjected to psychoactive extraction and his chances of survival are uncertain. Someone has stolen part of his mind and his attempts to resist have caused severe cerebral trauma. He’s weakening fast, so Romana asks Chris to collect the medical kit from inside the TARDIS; though shocked by the size of the interior, he does so. She tries to communicate with Chronotis, but the part of his mind that remains is now totally inert. Chris returns with the equipment and Romana attaches a strange collar to the Professor. He’s still breathing and his hearts are beating, so the collar can take over those functions and leave his autonomic brain free to think. Chris points out that the human brain doesn’t work like that, but Romana reveals that Chronotis isn’t human!

The Doctor has confirmed that the book appears to have no detectable atomic structure whatsoever. He believes the book had stored up a vast amount of sub-atomic energy and suddenly released it when the spectrograph was activated. It’s a very odd way for a book to behave and he concludes that it isn’t actually a book at all. Just then, the results of Chris’s carbon-dating test arrive and apparently show the book is 20,000 years old -- but the Doctor points to a minus sign that Clare missed, which means it is minus 20,000 years old. Not only is the book not a book, but time is running backwards over it!

Skagra returns to the Ship and tells the computer he was successful. He requests a playback from the sphere and sees images from Chronotis’s mind. He begins with the moment he attacked the Professor in the study, then rewinds to earlier memories. The images are indistinct and Skagra realises Chronotis had great control over his own mind, but his efforts to resist will almost certainly prove fatal. Skagra decides to find the Doctor because if the Professor didn’t have the book, then presumably the Doctor does, or at least knows where it is. The Ship detects the Doctor leaving St Simeon’s College and cycling back to St Cedd’s. Skagra is unfamiliar with the term "cycling" and the Ship explains that it’s a very primitive form of transport used on this planet. She recommends that he use the stolen automobile to intercept the Doctor at the riverside, so he picks up the sphere and prepares to leave.

Romana asks K9 to scan for any sign of conscious thought from the Professor, but it’s far too early to tell. Chris is starting to realise that a lot of things he thought were impossible are actually possible. He’s realised Romana is not from Earth either, but before she can explain her origins, K9 reports that Chronotis is deteriorating rapidly and there’s nothing they can do now as the condition is terminal. He can detect minimal cerebral impulses, so although Chronotis can’t talk, he might still be able to hear them. Romana asks if K9 can amplify his heartbeats, and she’s delighted to discover that the Professor is beating his hearts in Gallifreyan Morse code. She reads back the words “beware the sphere, beware Skagra, beware Shada”, but before he can finish his message the heartbeats start to fade and K9 concludes he is dying. At that moment all life function ceases -- the Professor is dead.

The sphere moves away from the Doctor as the TARDIS suddenly materialises in the street where the Doctor is lying. Skagra decides to withdraw and drives off with the sphere as the TARDIS door opens and Romana rushes to the Doctor’s aid. She helps him back into the ship and explains that K9 was able to trace the babble of voices. Unfortunately the Doctor has lost the book, so he asks K9 for an analysis of the sphere, but the machine is unable to identify its planet of origin. He plans to ask Professor Chronotis once they get back to St Cedd’s and is horrified to learn that his friend is dead. Romana tells him the sphere stole the Professor’s mind, and the Doctor is angry with her for leaving him while she was supposed to be looking after him.

In his study, the apparently dead body of Chronotis completely vanishes into thin air, making Chris jump to his feet in shock. The TARDIS rematerialises in the corner and the Doctor comes out and introduces himself to the young student. He accuses Chris unfairly of being responsible for everything that’s happened, then they realise they’ve now lost both the book and the Professor! The Doctor examines the area where Chronotis’s body disappeared and guesses he was on his very last regeneration. He remembers Skagra threatened to steal his mind too, but at least now he knows the name of their enemy. Romana and Chris recall that just before he died, the Professor told them to beware the sphere, beware Skagra and beware Shada. The name Shada doesn’t mean anything to them, but Skagra has now killed a Time Lord and a very old friend of the Doctor’s, so he thinks it’s time they had a little chat. The Doctor asks K9 to trace the sphere, but unfortunately the signal is far too weak to get a bearing so they’ll have to wait for it to become active again. They agree to wait inside the TARDIS, and invite Chris to come with them.

In the laboratory, Clare telephones Chris but gets through to his answering machine. She leaves a message telling him his teleprinter has provided more pearls of wisdom and she needs to speak to him as soon as possible. In the meantime, she decides to visit Professor Chronotis herself and consults the university directory to find out where he lives. Unfortunately St Cedd’s is right across town -- no wonder the Doctor wanted to borrow her bike! Eventually she arrives outside Chronotis’s rooms and knocks on the door. There’s no response, but when she hears a strange wheezing, groaning noise coming from inside, she decides to investigate. The rooms appear to be empty, so she calls out...

The Doctor fills the others in on the details of his recent encounter with Skagra. K9 then reports that the sphere has become active again and he’s tracked it down to a location nearly six miles away. The Doctor is grateful and wonders how he’s been able to cope without K9 these last few bodies. The TARDIS materialises in a field just outside Cambridge. The Doctor and the others emerge just in time to see the sphere floating up into the air and then completely vanishing. He asks whether anyone else saw what he didn’t see, then they head in the direction where they saw the sphere, carefully trying to avoid any cowpats. Inside his spaceship, Skagra asks the computer about the device the Doctor escaped into after their last meeting and learns that it displayed characteristics of a Gallifreyan time capsule, either a Type 39 or possibly a Type 40. It’s currently in close proximity and intruders have been detected approaching the Ship. The sphere recently returned with more minds, so Skagra realises the Doctor must have traced it back here.

As the Doctor explores the apparently empty field, he bangs his head on something he can’t see and realises there’s something invisible in front of him. K9 knew it was there, but didn’t warn anyone as he wasn’t aware that no one else could see it. He tells them it’s a spacecraft of very advanced design and many of its functions are beyond his capacity to analyse. Chris comments that if he had something that clever, he’d want people to see it! There’s not enough data to work out where the ship came from or how it’s powered, but they work out that it’s over 145 metres long. There must be an entrance, so they start to explore the outline of the ship -- then suddenly Chris bumps into what appears to be a ramp. A door opens at the top and when they enter, Chris is particularly impressed -- it’s much better than the inside of a police box! K9’s sensors are confused by all the signals he’s receiving so the Doctor thinks it might be better if the others all wait outside for him, but before they can argue they completely disappear before his very eyes. Skagra’s face appears on a nearby scanner and he assures the Doctor that his friends won’t be harmed... for the moment. The Doctor refuses to be impressed by such party tricks, so Skagra invites him to join him in the control room.

As they come face to face again, the Doctor asks him what he’s done with the Professor’s mind and Skagra informs him it will be put to a more useful purpose. He doesn’t care that Chronotis is dead as it was only his mind he wanted, and he accuses the Time Lords of having a very proprietorial view of the Universe. He refuses to disclose any information about himself, and instead produces the book and asks the Doctor to read it to him, but the Doctor says he has a very boring reading voice and that by the time he got to the bottom of the first page Skagra would probably be asleep. Skagra refuses to argue so the Doctor sits down and starts reading. Naturally the text sounds like complete gibberish, as it’s written in Gallifreyan code. Skagra orders the Doctor to decode it, but the Doctor claims to be too stupid. Tired of his resistance, Skagra lets the sphere loose on the Doctor and as it connects to his head, he collapses into his chair.

Romana, Chris and K9 are trapped inside a room with no door, having been brought here by some sort of matter transference. K9 is unable to pick up any trace of the Doctor as the room is shielded, so he suggests that Romana manually reconfigure his scanning wavebands. As she does so, Chris ruminates about a lecture he was due to give later tonight to the Astronomical Society in which he was going to finally disprove the possibility of life on other planets; he can still deliver it next month, although he’ll have to completely re-write it. The walls are made of a curious substance, and when the frustrated Romana shouts "blast it," K9 misunderstands and tries to blast it with his nose laser. The ray ricochets around the room, narrowly missing everyone as they drop to the floor. Just then, K9 starts to pick up some faint signals and he plays back the sound of the inhuman babble coming from the sphere. They notice it sounds different this time -- and K9 realises a new voice has been added... that of the Doctor!

Clare races across the courtyard at St Cedd’s and literally bumps into Wilkin. She apologises and asks where Professor Chronotis has gone. He doesn’t know but offers to pass on a message, so she tells him about the dangerous book. Wilkin thinks people shouldn’t really publish things if they don’t want others to read them, but she tells him the book itself is atomically unstable and is absorbing radioactivity. Although Wilkin doesn’t understand what she’s saying, he appreciates the urgency and sends her back to wait in the Professor’s rooms while he rings around to find the Professor.

K9 is no longer able to receive any signals on any frequency. Romana is frustrated and wishes she could get out of the locked room -- and then she suddenly vanishes in a beam of light. Chris is amazed and thinks he can do the same just by wishing out loud. It doesn’t work and K9 has insufficient data to explain why.

Outside the cell, Romana reappears before Skagra and demands to know what he’s done to the Doctor. He explains that he’s taken the Doctor’s mind and then drags her physically out of the spaceship. Accompanied by the sphere, they cross the field and head for the TARDIS. She refuses to let him in, but it doesn’t matter as he’s already stolen the key from the Doctor. Inside, Skagra tells Romana he can operate the ship himself using the information contained within the sphere. He programmes the controls and the ship dematerialises...

Clare paces up and down the Professor’s room in desperation. Eventually she decides to leave, but accidentally bumps into a cabinet. It opens up and amazingly, it’s filled with futuristic controls. She flicks a switch and all the windows automatically seal shut. An alarm sounds and the room itself starts to shake, as if in an earthquake. Then the sound of a TARDIS dematerialising fills the air... Moments later, Wilkin arrives outside the Professor’s door and calls out for Clare. There’s no answer, so he tries the door -- and finds a shimmering blue void on the other side. The Professor’s room has completely vanished!

Chris and K9 reappear inside one of the spaceship corridors, although they’re unaware how they managed to escape. K9 says it’s imperative they locate the Doctor straight away and prepares to blast open the door to the control room, but Chris suggests they try pressing the ’open’ button first. They race inside and the oxygen levels are restored immediately by Ship once their presence is detected. The Doctor starts to wake up, realising he’s been too clever by three-quarters, but insists that he’s still dead so the computer will continue to obey him. Fortunately the Ship is only programmed to obey instructions blindly, not to think about them. He then realises Romana is missing; it’s obvious Skagra has taken her somewhere, together with the book and a copy of the Doctor’s mind inside the sphere. The only good thing is that the Doctor deliberately avoided thinking about the code to the book just in case his mind was taken. He starts off for the TARDIS to follow the trail of their enemy, until K9 tells them the TARDIS has gone too!

Skagra boasts to Romana that everything is proceeding smoothly, but she tells him the main controls are booby trapped to prevent people stealing the TARDIS. He knows that’s not true as his sphere holds all the information he needs. He tells her not to come too close as the sphere can do far worse things to her than she can do to it. She can’t understand why he would want to steal the TARDIS when he has a perfectly good invisible spaceship of his own, but as he’s the designer of the ship he knows it has certain limitations and he needs Time Lord technology to find what they’ve hidden. He asks her if she’s ever heard of a man called Salyavin...

The Doctor wonders whether Skagra stole the TARDIS in order to travel in time -- but why would he kidnap Romana as well, since everything in the Doctor’s mind is already at his disposal? There’s only one thing Skagra doesn’t know -- that the Doctor is still alive -- but the list of things they don’t know is huge: they don’t know where Skagra has taken Romana, why he wants to book, or what he wants to do. That’s enough don’t knows to win an election. The Doctor commands the Ship to tell him where Skagra has gone, but she doesn’t have that information. Chris thinks they’re back to square one -- and the Doctor realises that’s exactly where they need to go if they want to find out who Skagra is and what he’s up to. He orders the Ship to take them back to the last place Skagra went to before arriving on Earth. The Ship tells him the location was the Think Tank, and although the Doctor’s never heard of it he thinks it sounds terribly grand so he’s keen to find out more about it. As the order doesn’t conflict with the ship's programmed instructions, she agrees to activate launch procedures and takes off...

The TARDIS lands inside Skagra’s command ship in deep space. Skagra is greeted by the Krarg commander, whom Skagra says has a rudimentary intelligence gleaned from various silicon-based life forms he cannibalised. It’s very big, very strong and very loyal. He advises Romana not to get too close as the rivulets running through the cracks in the creature’s masonry are white hot lava. Skagra shows her a hologram of the outside of the ship, which shows billions of stars that he says are spinning uselessly through the void, along with trillions of people spinning uselessly through their lives. He shows her another image of billions of atoms spinning at random, expanding energy, running down and achieving nothing. He tells her the one thing that stands against entropy and random decay is life; then he shows her yet another image where the atoms are arranged so they have meaning and purpose. And he believes the best place to find meaning and purpose is inside his own mind. Romana mocks him, but he believes she doesn’t understand because her own mind is too limited.

The Krarg commander says they will shortly be needing new personnel, so Skagra grants him permission to activate the vats. He invites Romana to watch as the commander approaches a series of 50 vats, each containing enough raw material to make an entire Krarg army. Skagra proudly boasts that he makes sure there’s at least one vat on each of his Ships as you never know when you might need a friend. The commander switches on the equipment and inside one of the vats a fibre optic wire skeleton lights up. Black crystals begin to form around the skeleton and before long a brand new Krarg has been created. It asks for orders and Skagra tells Romana that with an army like that, no one can oppose him.

The Ship tells the Doctor their journey will take 39 astrasiderial days, which is the equivalent of nearly three months. They’re already travelling at full warp drive, but they have hundreds of light years to cover. Chris thinks that’s an incredible speed, but it’s not nearly fast enough for the Doctor, so he arranges for the Ship to alter its own circuitry so he can introduce it to a few new concepts. Once his verbal reprogramming is complete, the Ship activates its realigned drive circuits and continues the journey. K9 tries to attract the Doctor's attention, but the Doctor ignores him, telling Chris that he’s constructed a primitive form of dimensional stabiliser by remote control so now the journey will only take a couple of minutes. The Ship comments that for a dead man, he’s extremely ingenious.

K9 continues trying to attract the Doctor's attention as the Ship approaches the Think Tank. The Doctor still has no idea what Skagra is planning, but they know they have to put a stop to it as mind control is the most horrible thing he knows. A physical threat can be fought, but once someone’s lost control of their mind, they’ve lost everything. Suddenly that thought starts to ring a faint bell for the Doctor, but he’s not sure why, although he suspects it would help if he knew who or what Shada was. K9 then interrupts and tells them there’s a large unidentified hostile lifeform at large on this vessel. His sensors indicate it was created on board their Ship as an automatic self-defence procedure while they were travelling, no doubt triggered by the Doctor’s modifications. He predicts that it will arrive on the command area in 21 seconds, and the Doctor berates K9 for not telling him sooner... The Ship is mere seconds away from docking at the Think Tank when the door to the command deck opens and a huge Krarg storms in and strides towards the Doctor and Chris. It accuses them of being trespassers and prepares to kill them, but K9 is able to hold it off for a while with its blaster. The docking bay is opened and the Doctor and Chris flee for their lives...

Skagra tells Romana that the Doctor’s mind continues to live within the sphere, and somewhere in there, he's convinced, is the code that will unravel the secrets of the book. She wonders what’s so important about the book and he tells her it’s actually the key the Time Lords use to imprison their most feared criminals, such as Salyavin. The sphere completes its analysis of the Doctor’s mind and Skagra is outraged to discover that he didn’t know the code to the book after all. It suddenly occurs to him that a Gallifreyan code would have to include the dimensions of time, so he searches the sphere for some more of the Doctor’s recent memories and discovers that time runs backwards over the book. He decides to run a little experiment in the Doctor's TARDIS.

Inside Chronotis’s study, Clare is confused by what’s happening -- and is even more shocked when the Professor appears and asks her what she’s been doing with his machine! He checks the controls and stabilises the room so that it’s no longer shaking. He makes her a cup of tea and tries to introduce himself, but he struggles over a satisfactory form of grammar to cover the situation of timelessness. Right now they’re both standing obliquely to the time fields, and he tells her she achieved this by activating the controls of his TARDIS. It’s a very old model that he literally rescued from the scrap heaps and he’s not really supposed to have it, but her timely mishandling of the ship meant she tampered with his own time field at the critical moment. He now regards himself as a paradox within an anomaly. Their first priority is to track down Skagra as he now holds the key to Shada, the ancient prison planet of the Time Lords. They’ve been induced to forget about it, and if Skagra is meddling with time control and mind transference, he can only be going to Shada for one particular reason -- or rather, one particular person -- and it’s imperative he be stopped!

The Doctor and Chris emerge onto the Institute for Advanced Scientific Studies, aka the Think Tank, and they hear the emergency announcement warning people not to approach as the station is under quarantine. They wonder what Skagra was doing here, but as they explore Chris asks how they were able to travel faster than light since Einstein states this is impossible. The Doctor is amazed that Chris understands quantum theory as well as Planck, Newton and Schoenberg, and tells Chris that he has a lot to unlearn. They find the main laboratory, and inside, they encounter a wizened ghostly figure which emerges from the shadows and begs for their help... Meanwhile, back on Skagra’s Ship, K9 is struggling to hold the Krarg back and calls out to the Doctor for assistance...

In the Doctor’s TARDIS, Skagra has conducted an experiment which proves time runs backwards over the book. As he turns the pages within the time field of the machine, it actually operates the controls. He believes turning the last page will take them to Shada! He returns to his command carrier and orders the Krargs to prepare to leave. He then tells Romana to get ready to meet one of the greatest and most powerful criminals in history, a man who the Time Lords have chosen to forget -- Salyavin, who will be the lynchpin to his plans!

The Krarg is burning and crackling, having absorbed all the energy from K9’s blaster, and it looks as though it’s going to explode at any moment. The Doctor asks Chris to distract the creature while he struggles to revive Professor Caldera and the other scientists, but there’s not enough time, so they have to flee and abandon the scientists. As the three of them race down the corridor back towards their own Ship, the Doctor regrets not being able to save the scientists -- he knows Caldera was already dead, but he wasn’t able to check on the others. K9 reports that the only viable human life signs on board are the Doctor and Chris, which is some relief to him. The Ship’s computer tells them the Krarg’s metabolism has reactivated the Think Tank’s power systems and the destruction of the station is imminent, so she initiates emergency escape procedures. The Doctor realises the Ship’s engines will not get them away in time, so he orders the computer to dematerialise using the reprogrammed circuits he designed earlier. The Ship disappears just as the station explodes spectacularly.

The Doctor regrets the loss of all those brilliant minds, but congratulates the Ship on learning some new skills. However, they’re still no nearer to finding Skagra and he’s not sure what to do next. He tries asking the Ship again, but she responds that her master did not reveal his destination to her. However, she does know about the carrier ship. The Doctor asks her to take them there instead. The Ship is starting to feel uncomfortable about following the instructions of a dead man, but he manages to convince her that Skagra will be anxious to pay his last respects to him. Being literal-minded, the computer accepts his argument.

Professor Chronotis is attempting to get his old TARDIS working again and is worried that the movement from Cambridge was just a dying spasm. They’re now jammed between two irrational time interfaces and time is actually moving away from them. Even if they disentangle themselves, he’s going to have to be very careful or he may end up dead again. Clare asks him about Salyavin and he tells her that he was a criminal, although the stories of his exploits were undoubtedly exaggerated as he was really just a brilliant but hot-headed young man with a peculiar talent. Chronotis is struggling with the repairs as the interfacial resonator needs two operators. Clare offers to help, but she doesn’t really have sufficient knowledge... and then a thought crosses the Professor’s mind. He tells her he’s about to do something and she must never speak of it again to anybody. He asks her if she knows what a certain piece of equipment is, and obviously she doesn’t -- but then he stares into her eyes and tells her to let her mind go blank. Then he asks her the same question and she immediately identifies the equipment as a conceptual geometer relay with an agronomic trigger and a totally defunct field separator. More importantly, she knows they can afford to dispense with it if they can get the interfacial resonator working again. They get to work...

Skagra is confused when the Krarg commander reports that his Ship has docked with them and Romana theorises that perhaps the Ship missed him. Fortunately he now has a full complement of Krargs at his disposal, created with rudimentary movement and understanding of basic commands. Romana wonders what he wants with an army of witless thugs, but he tells her they have an important part to play in his later plans. Just then, the door opens and Skagra is shocked to see the Doctor, Chris and K9 enter, accompanied by some Krarg guards. Skagra asks how he survived the sphere and the Doctor explains that it only looks for what it expects to find, so he just made it look for the wrong things. He accuses Skagra of wanting to take over the Universe, but he laughs at such a limited and childish vision. The Doctor agrees -- it’s a troublesome place, difficult to administer and as a place of real estate it’s worthless because there’s no one to sell it to. Skagra reveals that his purpose is to fulfil the natural evolutionary goal of all life. With the aid of the sphere, he plans to make the whole of creation merge into one single godlike mind -- the Universe will actually be him! The Doctor offers to discuss things over tea but Skagra explains that he’s ready to start and there’s nothing anyone can do to stop him. He orders them all to be locked up and for the key to be melted down, but the Doctor tells K9 to open fire and in the distraction he’s able to escape with Chris, although Romana is quickly recaptured. Skagra orders the Krargs to hunt them down and destroy them while Romana will accompany him to Shada. They prepare to leave straight away in the Doctor’s TARDIS.

The Doctor and Chris are soon lost in the corridors of the command ship and things get worse when K9 arrives to report that Romana is not with them. Chris thinks Skagra is mad, but the Doctor says that’s a matter of opinion and his own opinion is that he’s mad and infinitely dangerous. K9 detects more Krargs approaching and as the group moves back down the corridor they think they can hear the hum of the TARDIS. They turn a corner and find a door that’s inconsistent with the rest of the ship and wasn’t there when they passed this way earlier. It’s made of wood and has a little name-plate on it that reads “Chronotis, Professor J F”. K9 tells him the Krargs have spotted them and are converging on the area, and not for the first time the Doctor realises how much he’s missed having him around to point out the patently obvious. They have no choice but to go through the door--

--where they find themselves back in Professor Chronotis’s study. The Doctor, Chris, Clare, K9 and Chronotis are all amazed to see each other, but fortunately there’s a pot of tea available to calm everyone’s nerves. They hear the Krargs banging on the door, but the Professor isn’t worried and asks the Doctor what he thinks of his TARDIS. He admits that it’s strictly unofficial and he’s not really allowed to have one, so what better way to hide it than to actually live in it? Neither Chris nor Clare have any idea how they got here, but Chronotis suspects Skagra is heading for Shada. The Doctor is surprised to realise he’d forgotten all about the Time Lords’ prison planet -- but he does recall that Salyavin had unique mental powers and had the capacity to project his mind into other minds. With Skagra it’s quite the opposite; he has the ability to take people’s minds out of them, but he can’t put other minds into them. He’s obviously going to Shada to capture Salyavin within his sphere. It will make him omnipotent and able to move into every mind in the Universe so they’ll all work together like a single organism. It might take thousands of years, but he’d become immortal and spread like a disease. Chronotis suggests they follow the space-time trail of the Doctor’s TARDIS, which was the method they used to come here in the first place.

The Doctor’s TARDIS materialises on Shada and Skagra emerges with Romana. The book worked perfectly by programming the ship to bring them here and unlock the door to the prison. They hear a recorded message warning them that unauthorised entry is punishable by matter dispersion, but Skagra ignores it and when Romana protests, he orders the Krarg commander to keep her silent. He finds the prison index and passes over Rungar (who was sentenced for war crimes) and Sabjatric (the mass murderer), before he comes to Salyavin, who was imprisoned in perpetuity for mind crimes. The prisoners of Shada are all kept in their own separate cryogenic cells, alive but perpetually frozen. Romana refuses to comment on the morality of Shada, claiming that she isn’t answerable for the Time Lords -- but he reminds her that she’s their President even if she isn't responsible for previous administrations. In any case, the Time Lords will soon be irrelevant.

Not long after, Chronotis’s TARDIS also arrives on Shada. The Doctor orders Chris and Clare to stay behind while he explores with K9 and the Professor. Chronotis leads them in the direction that Skagra would have gone, and when the Doctor asks how he knows, he claims he heard footsteps. The Doctor considers retrieving his TARDIS so that Skagra would have no means of escape, but Chronotis believes it’s more important to stop him before he reaches Salyavin. K9 discovers the TARDIS is being guarded by two Krargs, so the Doctor has no choice but to go along with Chronotis’s strategy. The Professor instructs K9 to destroy Skagra’s sphere if he tries to use it on anyone else.

Chris is getting frustrated about being left behind and comments on how odd the day is turning out to be, but Clare thinks there’s something even odder about the Professor. When Chris complains that they’re being treated like idiots just because they don’t have the remotest idea how any of the equipment works, she surprises him by admitting that she understood everything, at least for a while after the Professor did something to her mind. She explains that Chronotis just walked into her mind as if he’d just barged through her front door and started shuffling things about. The Doctor told Chris earlier that this ability is unique to Salyavin, the man Skagra has come here to find... which means Chronotis is Salyavin! Chris rushes Clare to her feet and they race out into the prison.

Chris and Clare race to the scene, and before the Doctor can shout a warning, the sphere attaches itself to Chris’s head. Within moments, he’s possessed too and joins the universal mind of Skagra, stalking towards the Doctor like a zombie. During the distraction, Romana tells K9 to open fire, but this time not at the Krargs but at the walls to create a series of ricochets that will act as a barrier them and their attackers. It works, and the Doctor collects together his friends and they make their escape, regrettably having to leave Chris and Chronotis behind. Skagra realises the blaster fire cannot harm the Krargs and orders them to step through and bring the intruders back.

The Doctor leads Romana, Clare and K9 to the Professor’s TARDIS. They race inside and close the door, finally able to rest for a breather. They analyse what they’ve seen and realise that all Skagra has to do now is think of something and the zombie Krargs will do it. Even worse, the universal mind also contains the great intellects of the Think Tank scientists as well, and with Salyavin in the melting pot too, Skagra can now control anyone and everyone. Then Romana reminds the Doctor that his mind is also in there...

The Krargs begin banging on the door to the Professor’s TARDIS, but to no avail. Skagra joins them with several of the controlled prisoners he's released from their cells, but now he believes the Doctor’s actions are merely those of an insect threatened with inevitable extinction and he’s content to leave him there to amuse himself. Instead, he leads his group into the Doctor’s TARDIS and prepares to return to the carrier ship. From there, a fleet of small craft will take each of them to selected population centres and the great mind revolution can begin.

The Doctor has proposed a dangerous plan to Romana, and he warns Clare to hold onto something as he and Romana begin operating the controls to Chronotis’s TARDIS. The result is immediate and Skagra realises an external influence has taken over the controls of the Doctor’s ship. He opens the scanner and sees the Professor’s college room floating next to them in the space-time Vortex, generating a force field around them. Fortunately for him, the force field is getting weaker already and he estimates it will dissipate completely within two minutes.

The Doctor congratulates Romana, but they haven’t got to the hard bit yet. He instructs Clare on how to assist Romana, then warns them both to hold on as they’re in for a very rough ride. He starts making a speech to motivate them, but by the time he reaches point 23, Romana orders him to get a move on! He points to an area in the middle of the room and asks her to switch off the Vortex shields to create a small piece of timelessness and spacelessness just behind the tea trolley. It’s a difficult manoeuvre, but with care and precision, they’re able to achieve it and then the Doctor prepares to demonstrate a little trick he learned from an space-time mystic in the Quantocks. He steps towards the tea trolley and completely disappears...

Skagra admires the Doctor’s resourcefulness and realises he’s used the force field that was created by the two TARDISes trying to move through the same part of the Vortex to make a tunnel between the two crafts. But the force field is fading fast and Skagra is confident it will be a futile attempt. In Chronotis’s TARDIS, Romana is having the same worries and asks K9 to check out the sub neutron circuits. He detects a malfunction and he won’t be able to do the repairs in the time available, so instead he attempts to slow down the deterioration as long as possible. Clare is holding down a switch, but it’s starting to get very hot. Romana urges her to hold on, or else the Doctor’s force field will lose cohesion and he’ll be sucked into the Vortex. The switch is starting to burn, so Clare reaches for something to hold it down with, but there’s an explosion near the console and everyone in the room is thrown to the floor.

Skagra and his mindless prisoners struggle with the controls of the Doctor’s TARDIS and eventually they’re able to stabilise it. They make an excellent team, and now that the Doctor appears to have failed in his attempt to stop them, they can continue. In the other TARDIS, Romana and Clare wonder what happened to the Doctor as they weren’t able to give him as much time to make the crossing as he’d asked for. K9 predicts he has a 0.47 percent change of survival after the tunnel collapsed, so all they can do is go ahead with their plan as arranged. Unbeknownst to anyone else, the Doctor has materialised inside the workshop aboard his own TARDIS. He starts rummaging through the equipment and collects together what he needs, rejecting a rubber duck but choosing instead a crash helmet that Evel Knievel gave him. He pieces together the equipment using the sonic screwdriver and attaches it to the helmet. He’s going to look very silly, but even if it doesn’t work at least his enemies will be paralysed with laughter. He heads for the control room and hears the distinctive sound that means the TARDIS is landing.

Aboard the carrier ship, Skagra issues orders to his new army then tells the Krarg commander to start up the vats again so he can have more troopers. To his deep irritation, he’s interrupted by Chronotis’s TARDIS materialising, as K9 was able to complete the necessary repairs. K9 emerges and orders Skagra to cease his operations and surrender immediately, but Skagra urges the Doctor not to hide behind his metal dog and come out to face him properly. He’s surprised when Romana and Clare come out, and even more so when the Doctor then emerges behind him from his own TARDIS. Skagra announces that there’s no place for the Doctor in his new Universe and prepares to kill him, but the Doctor activates his crash helmet and turns to face Skagra. At that moment, everyone in Skagra’s army turns to face him too. The helmet not only protects the Doctor from the sphere, it also reflects back the brainwaves, giving him equal control over the zombie army. It’s now down to sheer willpower -- the Doctor’s against Skagra’s. Skagra urges his army to face the Doctor, but he only succeeds in controlling three of them. The Doctor reminds him that his own mind is contained within the sphere as well, and Skagra is straining too hard on the army to control the sphere too, so the Doctor is easily able to take control of it and pass it to Romana for safekeeping. The mental battle between them continues for a while, leaving the zombie army confused as to what their instructions are. They are no longer a universal mind following one master.

Chris returns to normal and is greeted warmly by Clare before he passes out. The Doctor orders K9 to fire upon the Krarg commander and the creature starts to absorb too much power. Romana instructs him to keep firing and drive the creature towards the vat areas, and just as it reaches them, it explodes, taking the entire annexe of the carrier ship with it. No longer able to create any more replacement Krargs, Skagra runs off, threatening to make the Doctor pay for what he’s done. The Doctor seems quite happy to let him go and says their priority is to go back to Shada to find Chronotis’s body. He congratulates everyone, then he takes his own TARDIS while Romana takes the Professor’s.

Skagra races back into his own Ship and orders it to take off immediately -- but then he suddenly disappears... and reappears in the cell area where Romana and Chris were imprisoned earlier. He demands to know why he’s been transported to the brig and the calm voice of the Ship explains that she can no longer follow his orders as he is an enemy of her lord, the Doctor. Skagra insists that he built her and is therefore her master, and he orders her to release him -- but she is more interested in telling him more about the Doctor. She says he’s done the most extraordinary things to her circuitry, and if Skagra wants, she can tell him all about how truly wonderful the Doctor is. In fact, she wants to start from the very beginning, and Skagra falls to his knees and screams...

On Shada, Clare is still worried about Chris, but Romana thinks he’ll be alright and he seems to be sleeping peacefully now. The Doctor returns and says he’s found Professor Chronotis, but he’s still in a coma and it’ll be a while before he gets his mind back. Now they have to decide what to do with him. The Doctor proposes leaving him here on the prison planet and letting the Time Lords sort it out as he has no wish to play judge and jury. He reminds Romana that Shada was only forgotten by everyone because Salyavin stole the book and made them forget in order to prevent his escape being discovered. Romana points out that it was Salyavin himself who contacted the Doctor about the book as he knew he was reaching the end of his lives, and she thinks it seems harsh to put him back into prison when all he wanted to do was make amends. He’s an old man and an old friend. The Doctor says that as she’s President of the Time Lords, it should be her decision whether he can go back to Cambridge. She makes her decision and they prepare to head back for Earth. He’s pleased to hear that, and is reassured that she’s a wise President who can see the bigger picture after all.

At St Cedd’s College, the porter Wilkin has reported the disappearing rooms to the police and he’s now escorting a constable back to the scene of the crime. The officer is rather sceptical about the story as it’s his experience that people don’t usually steal rooms; in fact, he can’t think of a single example of it happening. After all, what’s the point of stealing a room -- there isn’t much of a black market in them. Wilkin doesn’t appreciate the constable’s sarcasm, so the officer asks him to run over the salient points again for him. Wilkin reminds him that when he opened the door to Professor Chronotis’s room, there was absolutely nothing behind it except for some sort of blue haze. The constable thinks the ’blue haze’ might be the vital missing clue and Wilkin assures him he wasn’t drinking. They arrive at the door in question and the officer notices that it’s taken a bit of a beating recently. He knocks on the door and a female voice invites him in. It seems that whoever stole the room has decided to bring it back!

The constable and Wilkin enter Chronotis’s study and are greeted by the Professor himself, who’s having tea with the Doctor, Romana, Chris and Clare. The officer tells them he’s making a routine enquiry following a report that this room has been stolen. Obviously this isn’t the case, but Clare does take the opportunity to report her missing bicycle. Chronotis offers Chris some aspirin as he’s still recovering from the brain drain, which makes the constable suspect there might have been some ’celebrating’ going on last night. The usual sort of high jinks for students, such as roaming the streets and stealing bollards and policeman’s helmets. Just at that moment he spots the police box in the corner of the room and his worst suspicions seem justified. The Doctor claims that it belongs to him and he and Romana enter, saying goodbye to everyone and promising to keep the Professor’s secret (and his book). The TARDIS dematerialises. The officer is shocked, and when everyone denies any knowledge of the police box, he insists they accompany him down to the police station.