The Doctor is making repairs to the TARDIS, beginning with the scanner's audio system, although adrift in the silence of space is hardly the best test. A massive magnetic disturbance converges with the TARDIS forcing the Doctor and Nyssa to flee the console room. The field catches up with them and merges with the Doctor: he collapses. When he recovers the Doctor comments that he was being taken over, just for a moment though. It was a temporal bonding, an attempt to realign two basically imcompatabile life forms. Nyssa is concerned that the creature is formed from anti-matter. If it fails to fully reverse its polarity and the creature attempts to exist in our universe then it could be the end of our universe. The creature is shielded and with the universe under threat the solution appears simple. When the TARDIS is summoned to Gallifrey the Doctor suspects that he is being brought home in order to put a stop to the creature's attempt to exist in our universe. To achieve that, it appears that the only solution is to destroy the Doctor.

Robin Stuart and Colin Frazer are wandering around Amsterdam. Having lost his passport, Robin convinces Colin to seek alternate accommodation - by hiding out in a cellar. During the night, Colin witnesses the appearance of a strange creature - it uses a device on him, causing him to vanish. When Robin awakens later, seeing the same creature he makes his escape. Without his passport, with nowhere to sleep, Robin is not sure what to do next. The only possible answer might be Colin's cousin who is arriving in Amsterdam shortly - Tegan Jovanka.

Tegan and Nyssa have been helping the Doctor with the TARDIS. However, when he discovers that the ship has not landed where it should have done he tries to recall which one of his travelling companions entered the co-ordinates. Further information about the planet they have arrived on, Manussa, indicate that it was once part of the Sumarran Empire - the Empire of the Mara. On discovering that Tegan has also has a recurring nightmare, the Doctor decides to try and help her by inducing a dream state. It becomes clear that some vestige of the Mara is still lurking in Tegan's mind. That vestige has brought them to Manussa for a reason. They must leave the TARDIS and investigate. Until they can find the answer though, Tegan must wear an anti-dreaming device.

Ambril welcomes the Federator's wife, the Lady Tanha and her son, Lon. In honour of the up and coming celebration of the Federation defeat of the Mara, Ambril has arranged all kinds of activities - though few of them interest Lon. A tour of the city does little to interest him: an encounter with Dugdale, a showman, presents Lon with an example of local colour though to his mind a shoddy little booth containing distorting mirrors which people find amusing is just another example of why he is bored. Their next stop is a cave where a number of early Manussan artefacts and drawings have been found. Some tell of the days under the Mara. Lon wonders about the Legend of the Return, that one day the Mara will return. Ambril dismisses it but the tour is gatecrashed by the Doctor who assures Lon that there is truth to the legend. This truth can be found in the mind of his assistant, Tegan. However, when the Doctor escorts Lon back to the cave entrance, Tegan has disappeared, having taken fright at a toy snake. As the Doctor and Nyssa try to find Tegan they discover that she is no longer wearing her anti-dreaming device. Is the Mara all set to return...?

Ibbotson, a student at Brendon School admires a classic car belonging to one of the teachers. Fellow student, Turlough offers to take him for a drive in it - "just to the end of the drive and back". However, Turlough takes the car outside of the school and onto the open road. Its all just a game to him, but the game turns sour when they nearly collide with another car and are forced off the road. Turlough experiences a dream, at first thinking that he is dead. In this dream he is contacted by a guardian who knows that Turlough is not from Earth. Stranded there, Turlough would like nothing more than to escape. he would do anything to get off Earth - and the guardian offers him a way to do so, in exchange for a simple task: to kill one of the most evil beings in the universe - the Doctor!

The TARDIS nearly collides with a ship caught in a warp ellipse, its temporal orbit extending taking it to and from Earth every six years. Materialising on board the ship, the Doctor, Tegan and Nyssa explore it and find that it appears to deserted. One explanation could be the missing escape capsule, which left for Earth six years previously. There is no telling when it may return, so the Doctor and his companions return to the TARDIS, ready to leave. Except, something is preventing the TARDIS from dematerialising. Checking the TARDIS instrumentation the Doctor dashes back to the capsule bay: it has returned from Earth. As the Doctor dashes back to the TARDIS, Nyssa and Tegan conclude that someone or something might have come back with it and might even be in the TARDIS. Sure enough, the capsule's latest user is on board the TARDIS - Turlough. The Doctor readily accepts Turlough's explanation that he boarded the capsule out of curiosity and then found himself on this strange ship. The Doctor also accepts Turlough's help in severing the link between the beacon on Earth and the ship which is blocking the TARDIS' dematerialisation process.

One of the teachers at Brendon School has had a lot to deal with: his car stolen; his car wrecked; one of the two students involved has gone missing and the other has run back to the school claiming that Turlough entered a silver sphere which disappeared. The teacher, the Brigadier, assures Ibbotson that solid objects simply cannot dematerialise.

Turlough is exploring the TARDIS: the guardian leads him to a control panel and instructs him to alter the settings. He just manages to close the panel when Tegan locates Turlough. She warns him about wandering off but when the panel springs open Tegan wonders what he is up to. Showing him to his room, Tegan then goes off to speak to Nyssa and air her feelings about Turlough: she neither likes him nor trusts him. Nyssa prefers to have a more positive attitude, that Turlough just needs time to settle in. Left alone again, Turlough makes his way to the console room. Under the guidance of the guardian, Turlough grasps the space time element under the console and tries to pull it free. Although he cannot remove the element, Turlough's efforts appear to do enough damage as the TARDIS alert systems activate. The Doctor and Tegan return to the console room: the rotor has jammed and dimensional instability is beginning to affect the TARDIS. Nyssa, refreshing herself with her scientific knowledge from Traken, was hardly likely to have carrying out any kind of experiment that could have such consequences. Nevertheless, the instability which is tearing the TARDIS apart is encroaching on her laboratory.

The Doctor manages to activate a safety cut-out which enables the TARDIS to merge with another, more stable structure. This creates a temporary exit from the TARDIS which the Doctor urges Nyssa to use before the instability gets to her. With the TARDIS structure stabilising again, the Doctor, Turlough and Tegan dash down to the laboratory. Telling Tegan and Turlough to stay in the TARDIS, the Doctor passes through the exit into the ship beyond. He locates Nyssa and together they try to find out more about the ship the TARDIS has merged with. Thinking she can hear Nyssa scream, Tegan dashes out through the exit. Turlough follows and then watches as the door to the TARDIS fades - now they are all trapped on the mystery ship. Two other people board the ship - Kari and Olvir, an advance party of raiders. However, when the raiding ship disengages and abandons them, Kari and Olvir are presented with a horrifying realisation of the exact nature of the ship on which they are all trapped.

Something is drawing on the power of the TARDIS, much to the concern of the Doctor. When he finally understands what is happening, the Doctor turns up the power and then proceeds to communicate with the White Guardian. Only receiving part of the message, including a set of co-ordinates, the Doctor is alarmed when the Black Guardian also appears, then the White Guardian disappears and the power levels return to normal. Turlough has turned down the power output fearing that the TARDIS would explode. The Doctor enters the co-ordinates which take them direct to wherever it is that the White Guardian needs them to be. The Doctor and Turlough leave the ship, to explore. Tegan is left behind in case the White Guardian tries to contact them again.

The Doctor and Turlough explore what turns out to the hold of a ship. On the deck above they find the crew quarters and pretend that they have just joined. From the crew they learn that they are on a racing yacht and that the race is due to begin shortly. The only problem is that the crew don't seem to be able to remember joining the ship, only signing on aboard. And then there are the officers - a very peculiar bunch indeed. One of the officers appears and the Doctor is summoned to be presented to the captain. In the captain's dining room, the Doctor finds Tegan - the White Guardian managed to pass on a further piece of information that the winner takes all and that whoever it is must not win. Unfortunately the energy demand caused an overload on the console and so that is all that was passed on. Tegan also recounts a strange encounter with Marriner, one of the ship's officers.

As the race gets under way, a number of other mysteries begin to arise: crew affected when they go up aloft for the first time; wet-suit like clothing outside the bridge; and a map of the race area where the marker buoys are far more than just that.

Lord Ranulf is playing host to King John: providing him with food and entertainment. Except, the king states that neither is to his satisfaction. His champion, Sir Gilles, throws down the gauntlet as an expression of the disgust felt by such an insult to the king. Ranulph's son, Hugh, takes up the gauntlet: tomorrow they will duel, to the death. Ranulf tries to intercede but neither Hugh nor the king will countenance such dishonour.

The following day, Hugh and Sir Gilles face one another on the jousting field. A blue box materialises on the field, interrupting the joust. The king welcomes the new arrivals, the Doctor, Tegan and Turlough, despite all around fearing these "demons". The joust resumes: Sir Gilles knocks Hugh off his horse; dismounts from his own; and is about to deliver the killing blow when the Doctor intercedes. The king accepts his counsel and spares Hugh - much to the shame of the latter. Ranulf and Isabella are grateful, though that gratitude soon wears thin when Sir Gilles has Isabella imprisoned, apparently on instructions from the king.

Ranulf's cousin, Sir Geoffrey arrives and is amazed to find the king there: he had left then king the day before, having taken the Crusader's oath before him. How can the king be in two places at the same time? And why is he so accepting of demons?

The Doctor, Tegan and Turlough are enjoying the peace and tranquility of the Eye of Orion. Taking advantage of the break, the Doctor has upgraded the TARDIS console. The peace is shattered when the Doctor suffers a series of seizures, as if parts of his past are being torn away. With the help of Tegan and Tulrough he manages to get back to the TARDIS and sends out a message to his other selves, before collapsing.

The first Doctor is taking a stroll through a garden when a mysterious object appears in the sky and scoops him up. Wandering around a strange series of metal corridors, the Doctor is delighted to find his granddaughter, Susan. However, delight turns to alarm when they find a lone Dalek in the corridors and it pursues them.

The second Doctor is visting UNIT to witness the inauguration of Colonel Crichton as its new CO. During a chat with the Brigadier, the Doctor notices a strange object in tke sky. Urging the Brigadier to run, the Doctor tries to escape from the object, but it scoops up both him and the Brigadier. The pair then have to elude a squad of Cybermen roaming the landscape they have arrived in.

The third Doctor is driving along in his car, Bessie, when he spots an object in his path. Despite some expert driving, the Doctor is unable to elude the object - it scoops him up, car and all. Continuining to drive along, the Doctor spots and rescues Sarah Jane Smith. Reunited, they set off to find out more about this strange place. Along the way, they too encounter the squad of Cybermen.

The fourth Doctor and Romana are punting along the River Cam when the object appears and scoops them up. However, a freak eddy in the time vortex results in them being lost in the vortex.

The Time Lords have managed to stabilise the fourth Doctor and Romana: but the other Doctors have all being removed from their time streams by a device long forbidden. Even more alarming is the fact that the Doctor's various incarnations have all being placed in a large arena on Gallifrey, known as The Death Zone. It too, is a forbidden relic. Previous attempts to enter the Zone have failed and so the Time Lords resort to a desperate measure. They summon the Master: they need him to enter the Death Zone and rescue all the Doctors.

The Doctor, recognising the Tomb of Rassilon at the heart of the Death Zone, must unite with his other selves, elude the traps within the Death Zone, reach the Tomb and find the way out. They must also find out who has reactivated the Death Zone and to what purpose. Could there be any truth to the legend that Rassilon discovered the secret of immortality and that it rests with him, in his tomb?