Liz Shaw thinks back to when she first decided to leave UNIT. She’s not sure she can find the precise moment as it’s not the sort of decision one makes overnight. In some ways she’d decided to leave even before she joined as it was hardly the job she imagined herself doing when she graduated. But then she met the Doctor and the question really becomes when did she first decide to leave him?

She first met Jean Baisemore when they were at Cambridge together in the 1960s and to begin with she hated her as she was one of those irritatingly loud girls with far too much hair. Liz admits that in those days she found nearly everything irritating and was a big prude. Jean changed all that. They were both scientists, but Jean used to write essays about things like life on Mars and would often get laughed out of the lecture hall. But she not only introduced Liz to the possibility of life in outer space, she also taught her about the wonders of a social life outside of her studying. She even bought Liz her very first mini skirt and an extremely large white hat that was considered trendy in those days.

The last time Liz saw Jean Baisemore was after she’d been with UNIT for about a year. The Brigadier had had an argument with the Doctor about the electricity bill after his experiments on the TARDIS nearly led to the Electricity Board cutting them off. When the Doctor stormed off, Liz was left alone in the lab for a few days, so she decided to catch up with her old friend at Cambridge.

They’d agreed to meet at the Copper Kettle at noon as they’d both spent many happy afternoons sitting by the window and bitching about all the tourists. Jean was late, and as Liz knew punctuality was never one of her finer points, she wasn’t worried at first - but eventually she decided to call her. There was no reply, so Liz caught the bus to Jean’s house in a little village just outside Cambridge called Oakington. She’d stayed there before when they were both taking their degrees and she used to enjoy the view of the airfield from the spare bedroom window. When she arrived she noticed the curtains twitching slightly, but when she knocked on the door there was no answer. Liz let herself in the back door using a spare key…

There were two uneaten meals in the kitchen, which was unusual for Jean. Liz picked up some unopened post, then went to Jean’s bedroom. She was immediately hit by a terrible smell and then she noticed the floor was covered with torn paper from Jean’s books, which Liz knew she always kept obsessively pristine. In the corner of the room she found the bloodied remains of Jean’s cat Puddles.

Liz rang the Doctor at UNIT and was transferred through to his mobile phone on Bessie. To her surprise, she discovered he was already on his way to Cambridge as the Brigadier had just told him about an intriguing mystery involving disappearing scientists. Liz was almost relieved to discover her friend Jean wasn’t the only person to go missing in the last fortnight and it never occurred to her that this might be the start of another alien invasion. She was sure the Doctor would save her friend because that’s what he always did. While she waited, she opened Jean’s post and found it was just the usual stuff - bills, a charity request and a dentist’s appointment card.

The Doctor dismissed Liz’s fears that Jean had also been eaten and said he thought the cat’s death had probably been an accident - perhaps Puddles had been sleeping behind the record cabinet when it was merged with the wardrobe. Liz was worried the creatures might still be in the house, but a thorough examination revealed only more mangled metal. As they drove away, the Doctor noticed a hole in the ground about the diameter of a golf ball. He guessed it was a tunnel that led directly to the foundations of Jean’s house, possibly even into the fireplace. Liz was amazed that creatures so small could have done so much damage.

Half an hour later they joined the Brigadier in another similarly wrecked house on Grafton Street that belonged to a cleaner. This spoiled the theory that the creatures were targeting the scientific community and, in fact, out of 15 reported missing people, only three were scientists. UNIT were involved because one of the first people to disappear was involved in a top secret governmental research project. The Doctor found a dental reminder card, identical to the one Liz had found at Jean’s house, and realised the connection between all the people was that they shared the same dentist. The surgery was located on Trumptington Street and it was one of a small number that serviced the whole of the University. They also discovered the cleaner worked at the University too, specifically at Newnham College.

The mention of Newnham College reminded Liz of happier days when she and Jean would go for a meal and regularly break their promise only to have one bottle of wine between them. Even though they were both responsible adults and both had seminars the next day, they’d often find themselves staying up until three in the morning, howling with laughter. On one occasion they had to be escorted off the premises by a policeman!

Liz offered to investigate the dentist and was handed over to the ’new boy’ Captain Yates, who drove her to the surgery. Inside, she approached the receptionist, but realising she couldn’t ask outright whether the woman’s boss was responsible for the disappearances or whether she was an alien invader, Liz was lost for words. The receptionist explained that there’d been a cancellation and the dentist, Mr Arnold, could fit her in for an appointment in ten minutes. As Liz waited, she became aware that she was suddenly tired and light-headed. The wall-panels in the waiting room seemed to glow and there was a sweet and vaguely familiar smell. Liz closed her eyes for a moment and fell asleep…

She woke up and found the receptionist watching over her, claiming she’d had a reaction to the anaesthetic. Liz was about to say her mother was waiting outside for her when she noticed her mouth was numb from the injection. Liz’s memories of the next hour or so were hazy, but she recalled panicking as she left the surgery, temporarily confused and unaware of where she was. She fainted outside a telephone box and the next thing she remembered was the calming voice of the Doctor.

Liz found herself in her former tutor’s room at Newnham College. The Doctor checked that she was OK and explained that she’d been gone for four hours. She’d been discovered by Captain Yates, who’d taken it upon himself to investigate the surgery with a small group of men. During her absence, UNIT had been alerted to an unusual suicide on a nearby railway line and found the dead man was none other than Andrew Cull, the cleaner whose house they’ve been in earlier. He’d apparently jumped off a bridge and been hit by a train, which had sliced him in half. The two halves of his body remained surprisingly intact and when the Doctor examined him, he discovered Andrew Cull’s body was almost entirely made of metal.

The Doctor was sure Andrew wasn’t alien, although an examination of his skull showed it was almost entirely composed of blue shiny metal which was certainly extra-terrestrial. It reminded Liz of the fused objects found back at Jean’s house. It seemed that the creatures had somehow absorbed the metal in the house and transferred it across into Andrew’s body.

Captain Yates had investigated the Trumptington Street Surgery but found no trace of either the receptionist or any dental equipment. He did find Liz’s handbag, but the UNIT pass was missing, which meant whoever was responsible knew they were onto them. The Doctor decided to take a sample of the blue metal to a laboratory at the University, but Liz was worried about what happened to her while she was unconscious in the surgery. She returned to the mortuary and noticed things had changed - when she last saw Andrew’s body, only the skull had been transformed into metal, but now it extended downwards and was making its way into the man’s chest. Andrew Cull might be dead, but the invasive blue metal was very much alive!

The creatures sprayed the bullets with blue liquid and started absorbing them, then when Benton threw down his useless gun, they started fighting over it. Out of desperation, Liz grabbed a fire extinguisher and fired it at the creatures - and they immediately stopped.

Sergeant Broadwick was taken, semi-conscious, back to Newnham and while he rested in a side room, the Doctor identified the creatures as Cybermats. He’d never seen this particular variety before and said the liquid they manufactured in their mandibles was a very invasive living metal. The primary function of the Cybermen is to survive which meant they didn’t destroy sentient life, they converted it, so the Cybermats must be acting as infectors. Liz had been suffering from a terrible headache and every time she looked at a light it got worse. The Doctor remembered that the curtains at Jean’s house had all been closed and wondered if light intolerance was one of the symptoms. This meant Liz was almost certainly infected. The Doctor examined her mouth and discovered she’d been given a filling that was a vivid shade of blue!

The Doctor assured Liz he could reverse the process before it even started. At the University laboratory, he’d discovered a compound, consisting mainly of phosphorus tribromide, that could slow the growth rate of the blue metal. Liz offered to be a test subject, but the Doctor preferred to use Sergeant Broadwick as he was in more need of the antidote. They watched as a Cybermat crawled along the inside of one of Broadwick’s legs, but when the Doctor administered the antidote, the creature stopped. The Brigadier promised to have more of the stuff made up immediately, but then Broadwick’s skin started to change from blue to black. After a few moments, the man’s leg completely dissolved leaving only grey ash behind. Liz fainted.

Liz didn’t remember much about what happened after that, but she later learned that she’d had some kind of fit, during which she’d tried to eat a large volume of the “Encyclopaedia Britannica”. While the Doctor tried to work on an improvement to the antidote, the Brigadier had been issuing orders over his walkie-talkie when the signal was cut off. Then Liz had apparently hit Benton over the head with a large chunk of wall and demanded to be taken back to Oakington. She even tried to grab Bessie’s steering wheel from the Doctor’s hands as they approached the airfield by Jean‘s house. The only thing Liz could remember was trying to dig a hole in the earth…then the ground collapsed beneath her and she fell down into a tunnel.

The first thing Liz recalled with any certainty was the Doctor jumping down into the hole behind her. A few feet in front of them, the dark earth gave way to peculiar glowing metallic walls. The Doctor contacted the Brigadier by walkie-talkie and told him they’d stumbled across evidence of a crashed spaceship in an underground passageway, but when the signal was cut off again, Liz passed out once more.

When Liz woke up, the Doctor told her she’d attacked him and knocked him out. Since he came round, another five people had arrived and he guessed they, like Liz, had been led here by some sort of homing device. They were both restrained in devices like upright coffins with thick transparent tubes through which liquid bubbled. Ahead of her, Liz could see a figure almost entirely composed of the blue metal, and although one of its hands and a single closed eye appeared to be human, the rest of its face was a smooth mask and the mouth was just a narrow slit. The human eye opened and the figure spoke to her with a monotone voice that she recognised as belonging to Jean Baisemore. The figure apologised to Liz and begged her to kill it. Then a tube emerged from above Jean’s head and embedded itself into her eyebrow. Two Cybermats scuttled through the tube and Jean screamed for the last time as the conversion was completed.

Liz was overcome with disgust and fear, but the more she struggled, the more the tubes around her arms tightened. She heard metal footsteps approaching, then the Doctor asked the new arrival about the crashed spaceship. The half-Cyberman refused to answer his question, but the Doctor said he might have been able to repair the ship if the damage wasn‘t too extensive. The Cyberman said the original purpose of their mission had been superceded and the planned invasion was no longer necessary. This was a scout ship sent ahead to recee the planet in advance of the main invasion force, but it crashed before they could make their report and they’d only recently woken up. The Doctor told the Cyberman that the invasion did take place after all, but it failed miserably. The Cyberman insisted that the conversion of life on this planet would continue regardless, thanks to their improved Cyber-insects.

The Doctor used his sonic screwdriver to release himself from the conversion casket, then he did the same for Liz. She recognised the sickly smell of the liquid that poured from the tank as the same smell from the dentist’s waiting room. The Doctor examined one of the other bodies in the coffins and although it remained more or less human, the lower part of the face had started to turn blue. There were at least 20 caskets in the corridor, but Liz crossed over to her old friend Jean and saw that she was identical to all the others. The tubes around the casket were moulding themselves around her arms and legs and blue liquid was being pumped directly into her body.

The Doctor led Liz away and they found themselves in another identical corridor, lined with more occupied caskets. Then they came across a small room like a pilot’s cabin, with a dead Cyberman at the controls. Although she was starting to suffer from a terrible toothache, Liz noticed the design was slightly different as this Cyberman had a chest unit and handles emerging from the sides of its head - and it didn’t have any legs! The door opened and the half-Cyberman they spoke to earlier ordered them away. The Doctor pointed to their captor’s legs and Liz realised that unlike the rest of its body, they were entirely metallic.

The half-Cyberman prepared to destroy the Doctor, but he argued that as a man of science he could be useful and it would be more logical if he were converted instead. He suspected the Cyberman didn’t actually understand why he was doing all this. He recalled that the Cyberman referred to his “pets” as Cyber-instects, not Cybermats, and he was starting to wonder if he was really a Cyberman at all. The Doctor believed this ship must have crashed years ago and the original pilot was killed. The ship stayed buried underground until their captor stumbled across it and found it full of scientific equipment that fascinated him. He started to experiment, beginning with the pilot’s legs, and somehow the living metal connected with him. The Doctor asked the half-Cyberman what his name was - and he reveals that it was Gareth Arnold. He said his experiments were successful and his plan was to create an army to conquer, convert, command and survive.

The Doctor suddenly grabbed Liz’s hand and pulled her through the doorway. They darted down the corridors until they reached another door, but as the Doctor used his sonic screwdriver, it was knocked from his hand by another Cyberman. Dozens more Cybermen stepped out of their caskets and started walking towards them. Then the half-Cyberman who was once Gareth Arnold arrived and the Doctor tried to convince him he could make him human again. He said he was sure Arnold was still driven by the one emotion that drove the Cybermen to become what they are - the will to survive. Arnold rejects this and says the Cybermen will survive because of the work he’d done and they’d grow stronger and function more efficiently.

Liz felt a sudden judder in her jaw and realised to her horror that her tongue was being forced to one side by something inside her mouth. There was an unearthly wail (Liz didn’t know if it was her, the Doctor or the Cyberman that made it) followed by the acrid smell of burning. Liz’s face felt like it was on fire and she passed out again.

She woke up on the sofa in Jean’s house. The Doctor greeted her and gave her what he said was a final injection. She asked what happened back in the spaceship and he told her that regrettably he had to put an end to Gareth Arnold‘s suffering. The Doctor didn’t have access to the improved antidote he’d just given Liz, so he had to use the original compound - but Arnold was too far gone. The moment he collapsed, the other half-converted Cybermen simply walked back into their caskets to await further orders. Liz asked about Jean, but the Doctor shook his head and said if he’d had more time he might have been able to save some of them. The Doctor pondered whether the living metal might have some medical benefits for humankind and he suggested to the Brigadier that some proper scientific investigation was called for. With any luck, Arnold’s legacy might still live on.

Liz recalled that Arnold was the dentist and she wondered what it was he put in her mouth. The Doctor handed her a handkerchief to reveal what remained of the blue metallic filling - and a tiny Cybermat! Liz felt sick and then realised she’d lost an entire tooth. She asked the Doctor why she’d tried to eat an entire book and he theorised that she had mirrored the behaviour of the Cybermat as it was growing inside her. It had a thirst for information, particularly about the structure and properties of metal, just as the ones who investigated the TV and the fireplace had.

They heard the Brigadier barking orders outside and the Doctor demanded to know what he was up to. Just then, Sergeant Benton arrived holding an enormous gun and the Brigadier explained that he’d received orders from Geneva to mop things up. The gun contained a barrel full of white fluid - the Doctor’s original compound. The Brigadier argued that the half-converted Cybermen were no longer human and were a danger to everyone, but the Doctor was horrified. A squad of soldiers descended into the underground tunnel and then moments later, they heard the shooting begin.