Evading security at the Pharos Project, Tegan, Nyssa and the Doctor manage to get to the TARDIS. The Doctor disappears into the TARDIS interior, leaving Nyssa and Tegan to figure out a way to rescue Adric. Their problems seemd solved when the Master's TARDIS materialises and stuns the security guards. Adric returns to the TARDIS and dematerialises the ship, taking them all away from Earth. He then sets off to find the Doctor, who is wandering the corridors of the TARDIS, searching for the Zero Room, a chamber that will enable him to complete his regeneration.

Nyssa and Tegan set off to find the Doctor and Adric: they finally catch up with the Doctor and ultimately they find their way to the Zero Room. As the Doctor enters a sleep cycle, Nyssa and Tegan receive a transmission from Adric - he has been captured by the Master! Outside the Zero Room, Nyssa and Tegan find that the TARDIS is getting warmer. They are able to interpret the information in the TARDIS databanks: the ship is travelling back into the creation of the galaxy. Trapped inside the in-rush of hydrogen, the TARDIS will be destroyed by the gathering energies and the consequent explosion. It looks as if the Doctor's new life is about to end very quickly...

The TARDIS materialises on an Urbankan spaceship, four days journey time from Earth. Monarch observes the Doctor and his companions exploring his ship, noting that they require an atmosphere to breath - one of the downsides of the fleshtime. Nyssa takes an interest in one set of instruments so the Doctor asks her to see if she can activate it and make use of it. A chat with a monitoring device appears to lead to an invite, as a door opens on the level above. The Doctor and Tegan accept the invite, Adric remains with Nyssa. from her friends. The monitoring device escorts the Doctor and Tegan to meet Monarch and his two ministers, Enlightenment and Persuasion.

Adric enters the TARDIS to check the ship's time curve readings against those on the device that Nyssa is working on. As soon as Nyssa is alone, Monarch has her isolated. Adric goes in search of Nyssa and joins the Doctor and Tegan. During their discussion, the Doctor tries to evade Monarch's questions about the TARDIS while Tegan provides some general information about contemporary Earth - amounting to a sketch of what people are wearing at this stage of Earth's history. Monarch invites the travellers to go to the refreshments area where they are reunited with Nyssa. Some of the human passengers on the ship come to the refreshment area and are introduced: Bigon, an Athenian; Kurkutji, an Australian aborogine; Princess Villagra, a Mayan; and Lin Fitu, a Mandarin. Two more humans arrive, both resembling the drawings Tegan supplied to Monarch of a man and a woman in standard Earth attire. They introduce themselves as Enlightenment and Persuasion, Monarch's two ministers...

While Nyssa takes the chance to recover from her recent ordeal, the Doctor, Tegan and Adrix explore the apparent paradise planet they have arrived on. Their exploration brings them to a set of wind chimes among the trees. The Doctor is intrigued by the pitching of the chimes and seems oblivious to Tegan being lulled to sleep by the sounds. Adric finds a man-sized device nearby and closes the door, which activates it. Its some kind of survival suit and it appears to have weaponry, so the Doctor and Adric are moved on by the suit as it automatically returns to base. The Doctor shouts out to Tegan for her to stay out of the way, however she has already fallen asleep under the wind chimes.

The suit arrives at its base, the Doctor and Adric meet the remaining Earth personnel there: Sanders, the expedition leader; Hindle - base security officer and Doctor Todd. The suit was used by another member of the team, Roberts, the third person to disappear. Oddly enough, Sanders is not overly concerned, believing that the three missing people have simply gone AWOL, seduced by the paradise-like world of Deva Loka. The Kinda, the natives, are non-hostile and quite primitive, although Todd points out that the necklace they all wear appears to be a representation of the DNA double helix.

Sanders decides that he will use the suit to go out and investigate Deva Loka himself, leaving Hindle in charge. The Doctor asks Sanders to keep a look out for Tegan - she is out there on her own and if there is even the faintest possibility that something sinister lurks within the forest, she might need help. Todd has concerns that Hindle cannot handle the stress - this seems confirmed when he somehow gains control over two Kinda men imprisoned in the base and then begins to act on his paranoid delusions.

Tegan has a dream in which she watches two people playing chess. Both players have a snake tattoo on their arms. A third person arrives, after the two players disappear into the darkness. He also has a snake tattoo on his arm and makes Tegan an offer. To help her consider his offer further he presents Tegan with a challenge: she is introduced to herself and must decide which one of the two of her is the original Tegan...

Shooting stars above the skies of England in the late 17th century prove to be herald of something more sinister. Plague begins to sweep across the land and villagers become suspicious of any outsiders. The Doctor and his friends are rescued from one group of villagers by Richard Mace, a man of the road. He takes them to his current refuge, a barn. The Doctor notes that Mace is wearing an alien device around his neck, an artefact he found in the barn. A quick search locates other artefacts and the Doctor concludes that the shooting stars were probably the fragments of a ship breaking up on entering the atmosphere. The artefacts, some of them fragile, suggest that there may be survivors. Deciding that he should offer them some help, the Doctor asks Mace to take him to meet the owners of the barn.

The house appears to be empty and there are signs of weapons fire - both current weaponry and more advanced weaponry. The evidence seems to be mounting up that the alien visitors may not be as friendly as the Doctor had hoped. Noticing that part of the house does not correspond with what he expects, the Doctor locates the alien's hiding place and soon discovers that they plan to make Earth their world. The plague is the means by which they will wipe out the human race...

The TARDIS materialises on the platform of Craneleigh Halt, England, 11th June, 1925. A chauffeur spots them and informs them that he has been waiting for them: Lord Cranleigh is expecting the Doctor and he is needed for the local cricket match. Taking advantage of this apparent good fortune, the travellers enjoy first the cricket match then a masked ball at the Cranleigh mansion. Throughout the day Nyssa meets with quite a few surprised looks: all is explained when she is introduced to Ann Talbot, they resemble one another.

As the ball proceeds, the Doctor takes a shower before donning his harlequin costume. Nyssa and Ann agree to have fun with family and guests by wearing the same costume, making it difficult to tell them apart. Tegan enjoys a few dances, especially the Charleston and Adric helps himself to food from the buffet. Everything seems to be relaxed and fine, but Cranleigh manor hides a secret, a secret which begins to unravel when a man is murdered and Ann herself is nearly killed, by a man dressed as a harlequin.

A squad, led by Lieutenant Scott, responds to a call from Professor Kyle. Her team of geologists and paleontologists were attacked in some caverns. A scan of the caverns from outside shows no life. It appears that of the team of eight, Kyle is the sole survivor. There can only be ony way to be sure and to deal with the attackers in the cavern - by going down there. Scott asks Kyle if she is willing to show them way. Kyle agrees: Walters is to remain on the surface and maintain a constant scan; Snyder is to stay with him. Kyle leads Scott and his squad through the caverns. Lights flicker and strange sounds affecting the communicator also cause the scanner to flare up. Walters cannot explain either of them, nor can he explain a new reading which has just appeared. Somewhere close to the cavern where Kyle and her attackers were attacked, three new signals have appeared.

The Doctor has emerged from the TARDIS after a blazing row with Adric. Tegan and Nyssa have joined him, hoping that the cool air of the cavern and their words will calm the Doctor down. His concern is that Adric wants to return to his own universe, and it is no easy matter. As the parties calm down, Tegan notices the cavern around them - rich with a natural phosphorence and dinosaur bones. Further on, they spot equipment and before the Doctor and his friends can get back to the TARDIS, they are surrounded by Scott and his squad. At least four members of his squad have been attacked and he has heard nothing further from them. To all appearances, the Doctor and his friends are the most likely suspects for the deaths of Kyle's survey team. What can be in the cavern that someone would kill to protect?

Promising his companions a trip to the Great Exhibition at the Crystal Palace in the mid-19th century, the Doctor actually succeeds in bringing Tegan home to at Heathrow airport. However, this occurs as the result of a near-collision with something travelling along a time contour. The Doctor makes himself available to the authorities at the airport and they are advised to offer him every assistance. Another concorde will take the same flightpath as the one which disappeared, the object the TARDIS nearly collided with. This time, the TARDIS will be aboard the second concorde. Sure enough, the second concorde follows the flightpath and it too disappears.

To Captain Stapley and his co-pilots, everything appears to be normal. The Doctor warns them that his readings show that the second concorde has followed the first and travelled far back in time. When air traffic control contacts them to guide the plane down it appears that the Doctor is wrong. Once the plane has landed though, Nyssa glimpses something and the Doctor starts to realise that nothing is what it seems. Convincing Stapley, Scobie and Bilton to concentrate, the Doctor shows them the reality behind the illusion. They are no longer on the runway at Heathrow but on some plateau. Nearby, is the other concorde. According to the Doctor they have all been brought back in time some one hundred and forty million years. By whom or what and for what purpose - that is something they will need to investigate.

Matters are complicated further when they meet the passengers and crew of the first concorde - they all believe they are at Heathrow. All of this seems to be the plan of a mysterious being who calls himself Kalid.