The year is 1970. A young girl called Sarah is walking along the boundary of a disused naval base in St. Oswald beyond a sign which clearly says "Ministry of Defence. Keep Out" She stops as she hears a strange electronic noise. She wanders closer to the base and is seen by Tom Ferris, a farm labourer. Ferris calls out to her but she is mesmerised by the strange sound. She feels out to an invisible barrier that appears to be set between a series of redundant concrete fence posts... and suddenly vanishes into it! The labourer stumbles disbelievingly to the remains of the fence but finds nothing to credit what he has just witnessed save a discarded handkerchief.

Jean is having a nightmare about Simon and Liz. Her nightmare appears to be connected to events in 1940. Liz cannot accept that the young naval rating could be her father and that the place they are in is the derelict naval station at St. Oswald's, but in 1940 when it was a research centre under the command of Mr Traynor. She questions young Frank further and offers some facts about where and when he was born. To Frank's astonishment, she is right! At the Bull Inn, Skinner is wandering about. He chances upon Traynor in the lounge.

An angry Gottfried takes Simon and Liz back to the Operations room. On the floor is the unconscious Frank. Traynor and Skinner are still arguing outside the naval station. Gottfried asks Liz what is wrong with Frank. He checks the supply room for an explanation but finds none. He tells Liz to look after Frank in the Common room. Simon is fiddling with the radar equipment when Gottfried sees him and tells him to stop, believing that Commander Traynor has told him to sabotage it. He orders Liz to fix them all some food whilst Simon tells him all he knows about the equipment. Graz and Fritz take the unconscious Frank into the Common room where Commander Traynor is also imprisoned.

With Gottfried occupied with trying to find whatever it was that Sarah said she saw being buried and Simon helping him, Liz forces Sarah through the time barrier. When she tries to follow, she gets her sleeve caught on the fence and struggles to free herself. Suddenly, she is spotted by Fritz who raises his gun to strike her but Gottfried intervenes giving Liz the chance to escape through the hole in the fence. Gottfried calls for her to come back but she is gone.

Liz is struggling to find the time barrier. Gottfried has opened up the skirting board and is struggling to find out what is behind it. Liz becomes panicky has she realises that the time barrier has vanished. Gottfried replaces the skirting board.

Jean is watching horrified as the events in 1940 unfold in her mind's eye. Simon is tormented by Graz and a struggle ensues. Frank grapples with Graz as Liz and Simon head for the window to escape. Graz fires at Frank then turns to Liz and shoots her at point blank range in the stomach. Gottfried enters and Simon tells a shocked Gottfried that Graz has killed her. Jean faints.

Liz and Simon emerge from the time barrier not in 1970, as they expect, but in an icy landscape. In the biting cold, they both collapse. Back in 1970 Jean is sharing the experience and is shivering. Frank notices this but Traynor comments that she isn't actually cold. Franks wonders what has happened to Liz and Simon and says that in order to return to the present they only had to crawl through the hole in the fence. Traynor states that they're still learning about the time barrier. A red-suited figure crosses the icy landscape and makes an examination of two large ice blocks, which have gauges on the side. The figure notices the two unconscious figures and lifts Liz, carrying her back across the wasteland to a sliding door that lets into a futuristic-looking installation. A woman meets the figure and he explains that there is 'another one outside.'

Liz is still looking at the family photograph when Jean enters the office. Jean is initially cold towards Liz but soon comes to realise that the girl she saw earlier really is her young daughter. The family reunion is broken up by the sudden entrance of Beth. She is irrationally angry that Liz has returned to the Ice Box. Jean explains that it is not Liz's fault that she has returned through the time barrier. She adds cryptically that Beth was once able to do the same. Then to a horrified Liz, she explains that Beth is in truth Liz's future self. It's the trick that the time barrier has played on them!

The Director, furious to find Simon in his office, makes a grab for him and attempts to throttle him. He suddenly begins repeating his speech and then halts as if frozen, allowing Simon to escape his grip. Dr. Bukov enters, without knocking, and the Director apparently recovers, ignoring Simon. He immediately launches into a criticism of Bukov's handling of that morning's power failure, claiming he had been careless and that the failure was due to human error.

Dr. Joynton is struggling to reach a switch set into the wall. Beth sees the children trying to escape the office when an alarm klaxon blares out. All of them head for the Fantasy room where Dr. Bukov has discovered the prone form of Dr. Joynton. He turns her body face up and then gently closes the late Dr. Joynton's eyes, pronouncing her dead.

Simon and Liz enter the lobby of the Ice Box. Simon hands Liz a survival suit and tells her to put it on. In the Computer Room Beth indicates that the temperature is now normal again. Dr. Bukov notes it is another breakdown in the standard procedures of the base's systems. Beth is now also curious as to the cause.

Simon and Liz exit the time barrier and find themselves back in the Ministry Field in 1970. As the two make their way back home, Simon reluctantly finds himself thanking Liz for returning to find him. Liz says that she didn't. Simon tells her that they must prevent the future they have seen from happening.

Vera is gently watering her small plot of vegetables when a group of raggedly dressed children enter the village. Vera asks one of the boys how they have done. One of them, Paul, replies that it gets harder every day and shows her a poor collection of foraged vegetables and fruit. Vera tries to cheer him up and shows him the small crop of cabbages she has been tending which appear to be doing quite well. Paul looks up to the sun as it beats down.

The old man ........ proceeds to unravel one of the spools of tape on a nearby computer bank. He snips a section of the unravelled tape and pockets it before attempting to re-attach the two cut ends. Simon sees this and creeps up on him. As the old man continues with his apparent act of sabotage, Simon asks him what he is doing. The old man, startled, turns to face the boy. His face revealed to him, Simon can only utter the name of the man he now recognises

As Simon and Beth try to comfort a distressed Vera, a heavy branch falls from a tree. Beth comments that 2957 and the technocrats have finally done it. "They're burning us up!" In London at the Ministry, Controller 2957 is in a meeting fielding problems with Alpha 4. The temperature rise together with a report on the level of the River Clyde is being relayed to him. Alpha 4 points out that 1957 was concerned when the first rivers were diverted as part of the Master Plan and that he was at the time assistant to Commander Traynor. As one of the Alpha secretaries relays a report on the Wye Valley, the video scanner looses reception.

n the computer room, Alpha 4 is still watching the record tape. Commander Traynor apparently betrayed the Master Plan by holding up the progress of the diversion of the northern British rivers. Before the tape can conclude, Simon who is still listening from the secret room accidentally causes the door to open alerting Alpha 4 to his presence. Alpha 14 goes in to investigate and sees Simon. Simon shines his torch in 14's face. 14 staggers and collapses. Breathless, Simon escapes from the secret room.

Beth and Liz have found their way into the Ministry and into 2957's office. Liz tries to uses the video screen to find Simon. Flipping between locations she eventually brings up an image of Simon and 2957 in their cell. They are being served food by one of the Alpha secretaries and a Delta guard. Whilst Liz and Beth discuss ways of finding their actual whereabouts, they fail to notice Alpha 4 enter the office.

Alpha 4 refuses to acknowledge Traynor's authority calling him a traitor to the Master Plan. He seems to believe more, 2957 however Traynor is quick to describe 2957's duplicity. Liz tries to defend him but Traynor dismisses her "lies" and taunts 2957. As Alpha 4 admits that he only came out into the jungle to gather evidence against 2957, he is faced with the quandary of who to believe... 2957 or Traynor

Simon and Liz struggle through the dying jungle on their way back to the time barrier. Liz stops to catch her breath. Simon tries to coax her to move on. She reflects on whether Beth and her community will survive with the caves as their refuge from the heat. With Simon helping her to her feet, she wearily moves on. Suddenly on the ground, she is shocked to find a dead bird. Simon picks it up commenting on the fact that wild life is starting to die off.

Simon is at the Ministry of Forward Development in Whitehall looking for Commander Traynor. The doorman is adamant he can't enter the building without a pass. Spotting Miss Stebbins in the corridor, Simon brushes past the doorman and pleads with her to see the Commander. Miss Stebbins explains that Commander Traynor is at 'R1' but she refuses to go into further detail about what 'R1' is because of the Official Secrets Act.

Liz and Simon make their way back to the window, through which Simon climbed in, when he first arrived at R1, but it is now locked. Simon pushes Liz into the room he had originally hid in and closes the door on her. Liz is shocked to see the mannequin that looks like her. Simon goes to look for a way out. At that moment an alarm is sounded.

Commander Traynor turns to face Devereaux and Frazer only to discover he is now alone in the monitoring room. He walks across to the door, which he finds is locked. A hissing sound fills the room and Traynor glances upwards to see a nozzle pouring out a thick cloudy vapour. He begins to choke as the gas reaches him.

As Liz and Simon pass the time chamber room, Simon realises that Liz's bracelet is still in there. They must retrieve it otherwise Devereaux and his people will know they have been in there. Luckily for them the time chamber door is still open and they enter the regency-style room. Liz hunts round for it and sees it dangling on the back of a chair. Before she can pick it up, Simon grabs her. He reminds her that they were hiding on the floor when it was lost. Why is it now on the back of a chair?

Simon reaches out for the time barrier. He finds it. Liz tells him she is scared of what lies beyond the barrier and where they might end up. Even if they do get back, how can they stop what is going to happen. Simon explains that it isn't Traynor who affected the world but Devereaux. The Devereaux in the Ice Box wasn't a real clone. He was a projection of one. The real Devereaux is the cause of the burn up and not Traynor. Liz is confused and needs more explanation but Simon drags her into the time barrier before he can explain further.

Jean is staring into the time barrier. She can see something. Frank asks her what. She isn't sure but it isn't Liz. Suddenly, she realises that it is Traynor himself. As Traynor become more and more agitated by the contents of the file, he is struck by a shaft of sunlight through the office window. Reeling from the brightness, he crosses to the window and closes the curtains plunging the room into darkness. He turns, terrified at the realisation of his true nature...