The Doctor is supposed to be looking for the TARDIS manual in his ship's library, but he's more interested in what's actually there than in what he started out looking for, such as a copy of Frankenstein and a signed first edition of The Murder of Roger Ackroyd. Then the TARDIS encounters another distraction; a glitch in the Vortex, in which a time ship is crashing over and over again, eternally repeating the last few moments of its existence. Even as the Doctor watches, a flock of vortisaurs swarms over the time ship, but the Doctor drives his TARDIS forward, scattering them. While he's at it he tries to nudge the other ship a few seconds on, out of the loop, to give its perpetually dying crew peace at last. However, he misjudges the impact and hits too hard, and as he tries to pull away to safety, the vortisaurs close in on his damaged TARDIS...

October 1930. The British airship R101 sets off on its maiden voyage to Karachi, ushering in a new age of lighter-than-air travel... in theory. In practice, Lieutenant-Colonel Frayling is concerned about the modifications imposed upon his design team by Lord Tamworth, Minister of the Air, and fears that the rush to get the R101 into action has compromised its airworthiness. Tamworth refuses to explain the rush, or what's in the sealed compartments on the ship, or who the unnamed passenger in cabin 43 is. All he will say is that this mission is more than a simple maiden flight. As the still uneasy Frayling departs, Tamworth sends his South African "valet", Rathbone, to check on the health of the passenger in 43. As Rathbone heads for the cabin deck, he passes Chief Steward Weeks and orders a pot of coffee for later. He pays little attention to the young steward whom Weeks is escorting to the smoking room, which is fortunate for the "steward". For the real Simon Murchford is still in Cardington, and his place has been taken by would-be adventuress Charlotte Pollard...

As Weeks and Charley approach the lounge, the R101 lurches momentarily -- which may be due to the TARDIS materialising aboard, pushing its way out of the Vortex just ahead of the pursuing vortisaurs. The Doctor emerges to find himself in an enclosed space with water lapping about his shoes, and he climbs a nearby ladder for a better look around. By the time he works out that he's in an airship, it's too late; Frayling has noticed the list to starboard and ordered that number 3 ballast tank be flushed again. The TARDIS is jettisoned from the airship, and the Doctor finds himself stuck on board, facing the prospect of a long cross-country journey to recover his TARDIS. Unless it was dropped out at sea.

Frayling reports to Tamworth that a storm front is approaching, but again Tamworth dismisses his concerns and refuses to postpone the flight. They are just approaching northern France, and the halfway point in Egypt is at least a full day's flight away... and yet Tamworth claims that they will make their rendezvous in an hour. As Frayling tries to understand, there is a screeching sound from outside and the entire airship rocks as though struck by something -- and in the confusion, Charley's disguise slips. She admits to the infuriated Tamworth that she got Murchford drunk and took his place for the thrill of the adventure. She bolts before anyone can stop her, and Tamworth sends Weeks after her -- threatening, quite seriously, to have her shot. This mission is far more important than he's been letting on; but there's another threat more serious than Charley which he knows nothing about. Frayling catches a glimpse of it in the lightning; something outside the airship, an animal like nothing he's seen before...

The Doctor finds his way to the cabin deck, where he hears odd, hoarse breathing sounds from cabin 43. Using one of Conan Doyle's old stethoscopes to eavesdrop, he overhears Rathbone threatening to shut his charge up with an injection. Before the Doctor can intervene, Charley runs into him, and he helps her to hide, delighted to meet a new friend -- as she must be if she's running from something. Weeks gives up the search soon enough, and Charley decides to throw her lot in with the Doctor, although she's puzzled by his enthusiastically mad behaviour and his casual claims to have met Lenin, Geronimo and even Empress Alexandra. When he peers out of a porthole, claiming that he's trying to work out their location so he can figure out where his time machine fell out of the airship, Charley becomes convinced that she's the victim of an elaborate joke. Something seems to be scraping across the outside hull, but the Doctor is more concerned with Charley's claim that the airship is now over northern France, heading for Karachi. A few more questions confirm the worst; the Doctor is aboard the R101, and if he knows his history, he knows that the airship will crash in flames in this very storm...

The Doctor and Charley hear Rathbone's cries and rush to investigate. Weeks has arrived with Rathbone's coffee, but he is unable to break through the locked door until the Doctor arrives and helps. Inside, Rathbone is struggling with a vortisaur, which has its head through the porthole and its teeth in Rathbone's arm. The Doctor throws coffee over it, driving it away, and somewhat blistering Rathbone's already injured arm. He realises that the vortisaur must have followed the trail of the TARDIS as it made its emergency materialisation, and it'll be back now that it's tasted blood; but for the moment he's more interested in the passenger. Why is he, or she, or it, sealed inside a deep-sea diver's suit? Rathbone, despite the pain from his arm, pulls a gun on the Doctor and forces him away, but Charley speaks soothingly to the passenger, calming it down. Rathbone thus permits her to remain while the Doctor and Weeks go off to deal with the vortisaur; when they return they'll discuss why Rathbone shouldn't have the Doctor and Charley thrown off the airship. The Doctor goes, taking some of the passenger's morphine and leaving Charley in somewhat dubious company. She's always wanted adventure, but not the kind that Rathbone has to offer. She spurns his advances, and he backs off, but that might just be because his arm is paining him. The Doctor had better return soon...

The Doctor forces Weeks to help him throw a table through the promenade window, shattering it and causing the airship to list badly to port. However, the gap is now large enough for the vortisaur to get through, and the Doctor attracts it to him by cutting his arm on a shard of glass and giving it the scent of his blood. Much to Weeks' amazement, once the vortisaur is through the window it simply starts lapping peacefully at the Doctor's arm, attracted by the scent of Time in his blood, and as gentle as a lamb once it's away from the pack. The Doctor has Weeks administer the morphine to it, knocking it out cold; now all he has to do is get it back to its natural habitat. But that requires getting back to the TARDIS, and first he has to deal with Charley and Rathbone, the mysterious passenger, and this broken window. First things first; he and Weeks lock the vortisaur in the galley, and go to the lounge to speak with Lord Tamworth. The Doctor knows him by reputation -- as will everyone when this day is done.

Concerned by the buffeting his ship is taking, Frayling urges Tamworth once again to abort the flight; however, Tamworth refuses and threatens to have Frayling arrested if he continues to obstruct the mission. When the Doctor arrives, claiming that he's just locked a monster in the galley, Tamworth is rather taken aback and demands to know if he's a spy. Without really giving the Doctor a chance to answer properly, Tamworth claims that he would have been rather insulted if the Germans hadn't tried to sneak a spy on board; and if the Doctor were an English civilian, Tamworth would have to have him executed, as they can't risk word of this mission getting out back home. The Doctor plays along, claiming to be an agent of the Zeppelin company, and while he's at it he offers to help see to the health of the passenger in 43 -- for he believes that the passenger is ill, and that Tamworth has a vested interest in keeping it alive. Tamworth agrees to escort the Doctor back to 43, but first gives Frayling another bizarre order; at 0030 hours, he is to ascend to 5000 feet, nearly three times higher than the R101 was ever designed to go. This is why Tamworth insisted upon the modifications last summer, but he still refuses to give his reasons. Frayling has no choice but to obey.

When the Doctor arrives at cabin 43 he pointedly informs "Fraulein" Pollard that their cover has been blown. Charley plays along. Rathbone's arm still aches, which isn't surprising; it was bitten by a five-dimensional predator and is now thirty years older than the rest of his body. But the passenger is in even worse condition, and something of its anxiety seems to be conveying itself to Charley, and even Tamworth. Part of their light-headedness can be put down to the changing air pressure as the R101 rises -- and the Doctor guesses that the passenger is even more sensitive to the changes. The precise oxygen mix required to keep it alive at ground level is killing it at this altitude. While Rathbone holds a gun to Charley's head, ready to shoot at the least sign of misconduct, the Doctor removes the passenger's helmet... to reveal that, as he'd suspected, it's anything but human. Rathbone is as revolted as ever by its alien appearance, but Charley finds it oddly beautiful; it reminds her of the dolphins at Regent's Park Zoo. The alien senses her kindness and reaches out to her, speaking her name -- but then an alarm goes off, indicating that they've reached the rendezvous point. Tamworth tells the Doctor and Charley to escort the passenger to the lounge; they've proven their usefulness, and while Rathbone may object, Tamworth's in charge. The Doctor and Charley wrap up the passenger in a warm blanket and place it in an invalid chair, watched by the suspicious Rathbone -- who is clearly far more than a mere valet.

Frayling is astounded by the saucer, which is over two miles wide and capable of flying through space. The alien seems better now that its people have returned, and it identifies itself as the Engineer Prime of the Triskele. Rathbone still objects to the Doctor's presence, but the Doctor forces him to admit that he is himself an agent for British Intelligence, here to ensure the safety of the passenger. It would appear that his task is now over. The Engineer Prime announces that the airship may proceed, and the hull of the Triskele ship becomes insubstantial to let the R101 pass through. Charley has always wanted adventure; she only stowed away on the R101 because the young trader who told her stories of his travels laughed when she impulsively told him she'd meet him on the terrace of the Singapore Hilton on New Years' Eve. But this is more adventure than she'd ever dreamed possible, and the Doctor is in his element. Everything he told her was true.

The R101 enters the darkness of the saucer's interior, but as Tamworth prepares to march out the guard of honour, the Engineer Prime gently -- but firmly -- tells him that this will not be permitted. Only three persons will be allowed aboard the saucer, and the Engineer Prime has chosen the three; Tamworth, Frayling, and the Doctor. Tamworth is deeply insulted by this apparent affront to the British Empire, but he's diplomatic enough not to show it too much, especially when the Doctor advises him to play along; why share the glory of first contact? Charley will have to remain aboard the ship, and although the Doctor isn't particularly keen on leaving her in Rathbone's company he has little choice. Tamworth privately admits that he too is concerned about what Rathbone might get up to if things don't go according to plan. He's right to be concerned. As soon as the delegation has departed, Rathbone orders the crew to unpack crates from the sealed compartments, and Charley's unease grows when she sees that the crates contain weapons and ammunition -- enough to fight a small war...

Once back in its own territory, the Engineer Prime can move without touching the ground. As the delegation proceeds, Tamworth explains that the Engineer Prime apparently crashed to Earth last winter. At first British Intelligence thought that the alien had been bred by a foreign power, and Tamworth is now rather contrite about the "intensity" of the interrogation. The breakthrough came when Rathbone contacted a spiritualist from Shoreditch, "Madame Zelda", who turned out to be a genuine psychic capable of making telepathic contact with the Triskele. The Engineer Prime claims that it had not communicated with its captors beforehand, for fear that this world was populated only by "Uncreators"; its opinion changed when it met Madame Zelda. Through Zelda, the British arranged to return the Engineer Prime to its people, and as they didn't want to risk authorising a public landing where the Triskele saucer could be seen, they decided to outfit the R101 to make contact in the air. This explains the modifications made to the airship -- but the Doctor doesn't think Tamworth's motives were entirely altruistic...

The delegation finally arrives at the Triskelion, a three-armed sigil which the Doctor has seen on many other worlds before. The Triskele are a very old race indeed, and were once feared across the Universe... The Doctor, Tamworth and Frayling each take a place on the sigil, and it seems to transport them through the ship at blinding speed -- but in fact, it's the ship which is remaking itself around the Triskelion. The Doctor is so excited that he can barely keep still as they arrive in the home of the Engineers, the part of the Triskele whom Frayling has been chosen to represent; the rational, thinking side of the race. But then the Triskelion takes them to the Uncreators -- the dark, twisted id of the Triskele, bound in chains by the Engineers for fear of what they could do. Or rather, un-do; for the Uncreators represent the Triskele's suppressed urge to destroy. The Uncreator Prime stands behind Tamworth just as the Engineer Prime stands behind Frayling; for Frayling is a builder, a designer, and Tamworth has the smell of blood on him. Tamworth admits to fighting in the Boer campaign, but he has no taste for war; he's seen too many young men shattered, their lives frozen forever in the horror of combat. The Uncreator Prime is furious, and accuses the Engineer Prime of cheating the Uncreators of their representative. It forces its way into the Engineer's thoughts, and senses that there is a true Uncreator aboard the airship... a man named Rathbone. Aboard the R101, Rathbone hesitates in his preparations, hearing a voice in his mind...

The delegates have only seen two parts of the tripartite Triskelion, and the Doctor has already guessed what the third must be. He's proven right when they travel once again, to an echoing void inhabited by only one creature, the Lawgiver. The Engineers are the creative, rational side of the race; the Uncreators are the destructive, emotional side; and the Lawgiver is the free will, the one who mediates between the head and the heart and decides what is to be done. When the Triskele were individuals they allowed their base desires to corrupt them, and their race committed atrocities throughout the Universe. To save themselves from themselves, they redesigned their species, dividing themselves into three parts; but whereas there are whole populations of Engineers and Uncreators, there is only one Lawgiver. And the Lawgiver is now very old indeed. When it dies, the Uncreators will be released from neurological bondage and run amok -- but they cannot select a new Lawgiver from amongst the Engineers or the Uncreators, for then the new Lawgiver would only represent half of the race. The Engineer Prime came to Earth in the hope of finding a new Lawgiver from outside, but it has failed; it was ordered to select an Engineer, an Uncreator and one other Earthling, but the Doctor is not from Earth and Tamworth has rejected his birthright of destruction. The Uncreator takes the opportunity to inform the Lawgiver that there is a more suitable Uncreator aboard the airship -- Rathbone. Tamworth, suddenly reminded of how much time has passed, realises that he must get back to the R101 at once. Rathbone has certain orders to carry out in the event of Tamworth's failure, and it's imperative that Tamworth stop him...

But Tamworth is too late. He's failed to return after half an hour and Colonel Peter Rathbone is about to carry out his instructions. Dismissing the voice in his head as a momentary lapse, Rathbone orders Weeks to escort the VIPs to their cabins, and to shoot Charley if she disobeys. Any failure to comply will be regarded as an act of treason -- and once the VIPs are safe then Rathbone will lead the airship's crew out into the Triskele saucer to take it by force. They haven't come all this way to return empty-handed, and the aliens must be made to respect the superiority of the British Empire. Charley and Weeks, knowing that Rathbone is making a grave mistake, follow him out into the airship, but the moment Rathbone steps out of the R101 he is transported to the Triskelion. There, the Uncreator Prime is greatly pleased by the violence in his mind, but the Doctor is appalled and disappointed when Rathbone reveals that they only came here for the Triskele saucer. Tamworth was in the delegation at Versailles, and he knows that the "war reparations" only postponed an even greater tragedy; with the technology of the Triskele in the hands of the British Empire he'd hoped to avert it all. Even he can see that this act of piracy won't bring peace to the world -- but Rathbone isn't exactly under his control any more...

Rathbone collapses, his mind broken by the Uncreator Prime's having used him as a tool. Weeks and his men open fire on the Uncreators, but for every one they shoot, another three take its place. But just as all appears lost, the enraged Tamworth drives off an Uncreator by shouting angrily at it -- and the Doctor realises that this generation has never seen another predator before. They're all instinct, including the instinct for self-preservation -- and thus, they fear creatures which appear more ferocious than they. The Doctor thus urges the humans to drop their weapons and approach the Uncreators, roaring and gesturing, making themselves appear as large and menacing as possible. He tells Frayling to think of all the times he's been put down and passed over, his advice ignored and his warnings scoffed at; he tells Charley to think of the Uncreators as smug, self-satisfied Singapore traders. But the Engineer Prime can't bring itself to join in; this may be necessary, but it's too barbaric for the Engineers to handle.

The Uncreators draw back, frightened off, but the furious Uncreator Prime has another trick up its sleeve. The triskelion on its breast is not just a symbol; it's been passed down through generations of Uncreators Prime, and the current encumbent unfolds it into an energy weapon with which it intends to kill the Doctor. Tamworth stands up to it, however, and declares his candidacy for the position of Lawgiver; if the Uncreator wishes to accept his challenge, he'll fight tooth and claw. Tamworth knows he hasn't a chance, but he's accepted his responsibility for what has happened, and he just wants to give the others the chance to escape. The Doctor urges Charley to take Rathbone and the others back to the R101; he has to remain, for he knows that the Engineers are too weak to resist the unchained Uncreators, and the new Triskele will spread cruelty and barbarity through the Universe if unchecked. But to everyone's surprise, Tamworth bests the Uncreator Prime; it fights by savage instinct, but Tamworth marries brute force to intelligence and gets the Uncreator Prime in a stranglehold. Desperate, the Uncreator Prime reaches out to Rathbone again and tries to force him to shoot Tamworth, just as he was influenced to shoot the Lawgiver. But Rathbone, struggling to resist the evil voice in his head, shoots the Uncreator Prime instead.

Tamworth has won. Now he has the finest spaceship in the galaxy at his disposal, along with a race of brilliant engineers and an army of thousands... but he knows better than to take them back to England, and deliver them into the hands of men like Rathbone. He will remain on the ship as it returns to space, not as the new Lawgiver, but as an advisor to help the Triskele restore their inviduality. The R101's engines have started; apparently the director of civil aviation has had enough and is preparing to leave. Tamworth asks only one thing of the Doctor before he goes... make sure the R101 gets down safely. The Doctor, clearly uncomfortable, promises to do his best, but as he and Charley return to the airship the Doctor tells her that she still has the chance to remain on the Triskele saucer. However, Charley has seen parachutes aboard the R101, and she urges the Doctor to board before it's too late. He does so, and the moment is lost; the R101 emerges from the Triskele saucer, which departs back into the depths of outer space, taking a very brave man with it.

As the R101 returns to a theoretically safer altitude, Weeks returns to the galley with some scraps of food for the vortisaur. It now appears quite tame, and Weeks is considering taking it home as a pet when this is all over. But there's something disturbing it -- and as tame as it might have appeared a moment ago, Weeks finds out the hard way that it's not a good idea to remain in a closed compartment with a very large, wild animal...

The Doctor and Charley return to the lounge, hoping to convince Frayling to evacuate the R101 before it's too late... but it already is. The Doctor taught Frayling to stand up for himself, and he's not returning to England empty-handed; for amongst the confusion, Rathbone retained the presence of mind to pick up the Uncreator Prime's energy weapon as it fell. The Doctor is furious; if they return to England with the triskelion they'll have energy weapons mounted on Spitfires in time for the Battle of Britain, and the empire which should be falling apart will continue on, changing the course of history beyond recognition. Although the Doctor abhors violence under normal circumstances, he has no choice but to sock Rathbone soundly and make off with the triskelion, intending to throw it out of the shattered window on the promenade. But as Charley follows they find the vortisaur blocking their way, and, pursued hotly by Rathbone, they're forced to detour into the very heart of the airship...

Rathbone corners the Doctor and Charley amongst the airship's great grey leather gasbags, which the Doctor warns him are leaking. The R101 is simply too large to fly -- and the vortisaur clawing at the hull hasn't helped its integrity. The gasbags can't cope with the buffeting of the storm winds, and soon, one will blow completely. The R101 will plummet from the sky, try and fail to right itself, and scrape alongside a French hillside, bursting into flames and killing everyone aboard. And since the airship is doomed anyway, the Doctor might as well fire the triskelion now, destroying it along with the R101 and preserving the web of Time. Charley believes that the Doctor is serious, but Rathbone calls his bluff and attacks the Doctor with an axe -- and misses, puncturing one of the gasbags. History is being made. As the airship begins to lose altitude, the Doctor throws the triskelion away, and Rathbone, trying to catch it, falls from the catwalk and plummets through a rent in the hull. It appears that the Doctor and Charley are about to go the same way -- until the vortisaur arrives, agitated by the disaster it can sense in the future, but still drawn by the scent of Time Lord blood. The Doctor used to ride vortisaurs bareback at the Academy, and with just a little manoeuvering he and Charley climb onto the vortisaur's back and ride to freedom.

They're the only ones. Frayling is in the lounge when the airship begins to fall from the sky. Desperate, he calls the flight lieutenant and orders him to increase elevation, but it's too late; the Doctor was right all along, and Frayling should have listened to him. Frayling opens one last bottle of champagne, and toasts the R101... and all the lost souls aboard. And the airship hits the ground, and explodes in flames.