The Doctor has regenerated but it hasn't gone well. He's mentally unraveling and needs a place to rest beyond outside influences. Unfortunately the Master has laid a trap for the TARDIS that denies the Doctor undisturbed recovery time.
To avoid falling into Event One (the Big Bang), the Doctor must convert mass into momentum by having the TARDIS jettison a quarter of itself. Unfortunately it can not be known which rooms will be ejected.
The Doctor discovers Castrovalva is a cursive occlusion, an impossible yet manifested Escheresque environment, with all pathways, no matter which direction one goes, leading back to the same point. It is a trap laid by the Master with no apparent way out.
Trying to transport Tegan to Heathrow airport on present-day Earth, The TARDIS accidentally lands on-board a alien spaceship traveling to Earth which will arrive in 4 days. Where the Doctor, Adric, Nyssa and Tegan encounter the frog like Monarch, ruler of the doomed planet Urbanka. Only to find Monarch has abducted generations of humans from different cultures and converted them into cyborgs. The Doctor and Tegan discover Monarch's true goal is to travel faster than the speed of light, and traveling back into time where he will meet himself and wipe out the human race...
The doctor figures that the Urbankans have visited Earth several times over thousands of years, with each visit coming closer together, but this time the Urbankans have come to stay. Bigone reveals how he and the other cultural representatives, taken at each visit, have lived so long.
Monarch shows his true plans for the humans when he sends Adric to ask the Doctor for a tour of his TARDIS while he sends Nyssa off to be relieved of her "flesh time." Meanwhile Tegan, in a panic, tries to escape in the TARDIS but doesn't get far.
The Doctor needs to stop Adric from siding with Monarch and get Bigon quietly restored (who was mechanically lobotomized for previously siding with the Doctor). The ship then gets turned into one big multi-cultural dance party so the Doctor can make a desperate leap for the TARDIS, floating just outside the spaceship, and stop Monarch's plans for Earth.
On Deva Loka, a sylvan paradise planet with no predators, diseases or civilized roadways, Nyssa (due to Monarch's two attempts to turn her into an android) stays in the TARDIS to fully recover from mild mental disorientation, under the Doctor's Delta Wave Augmenter while the others go exploring. The Doctor and Adric find a survey team assessing the planet for colonization, and cracking under the stress of three fellow members (half the crew) disappearing without a trace. There's also a primitive and almost entirely speechless native culture on hand who curiously have ...
Left in charge by Sanders, Security Man Hindle - unhinged and in complete control of the two Kinda hostages - locks up the others while he primes the dome for a 50 mile perimeter of deforestation through acid and fire, having "reasoned out" that the trees and plants are a threat. Adric perpetrates a ruse to help the Doctor and Todd but is caught and about to be punished when Sanders returns, a changed man. Sanders bears a gift for Hindle - a box given him by a Kinda mystic elder, which may be dangerous and which the Doctor is forced to open. Meanwhile, Tegan is shown ...
Todd and the Doctor receive a telepathic invitation to visit the Kinda's blind matriarch and her ward in a cave. Meanwhile, Hinkle is finally set to reduce the dome to its "base chemical constituents" but stops to relax and build a toy city. Tegan is out cold after the snake tattoo of the Mara leaves her arm to possess Aris, giving Aris voice and, through it, the ability to lead his people to their destruction.
With a frightening vision of what's to come from Panna (the Kinda's blind visionary), the Doctor must break Hindle's control over the Kinda hostages and stop his deforestation plan, then send the Mara back to The Dark Places of the Inside from whence it came before it can destroy the Kinda and head out to topple civilizations.
The TARDIS returns Tegan to Heathrow Airport: Tall trees, peaceful, clear skies, thriving vegetation and a touch of sulfur in the air. Obviously the TARDIS is a little out of date (about three hundred years or so), but upon looking around the Doctor finds there's more amiss than his navigational controls, all arising after a recent comet crash that was not on Earth's schedule.
To rescue Tegan and Adric, captured and interrogated by the Terileptil fugitive leader, the Doctor must get past the locals under alien control and overcome Death (an android servant dressed up as The Grim Reaper).
The Doctor makes the fugitive Terileptils a rare offer they shouldn't refuse; but, though few in numbers, they have a plan in the works for the quick and complete domination of Earth and mean to see it through.
With Tegan and Richard under alien control, and the manor sealed against anyone leaving or entering, it's a question whether or not the Doctor can get to London before the Terileptils release their improved strain of black death upon the world.
The TARDIS arrives in England, 1925, near to where a mysterious, gurgling man has escaped his bonds and started strangling servants. The Doctor is "expected" and taken (along with Tegan, Nyssa and Adeic) to the Cranleigh's country estate where The Doctor participates in a cricket match and Nyssa turns out to be the near spitting image of Ann, fiancée of Charles Cranleigh. They all stay for an afternoon costume ball. Tegan teaches others how to do the Charleston, and Adric eats, but The Doctor finds himself searching through secret passageways in his bathrobe after the...
Ann escapes her Harlequin-clad abductor but it's The Doctor whom she accuses. Strangled bodies mount and The Doctor is arrested along with "accessories" Tegan, Nyssa and Adric. Will Lady Cranleigh's secret come out?
In 2526, 8 Earth paleontologists and geologist were investigating a fossil find in a newly discovered case when 7 of them disappear. The only one who seems to have got out is Professor Kyle. She accompanies Lt. Scott and his troop of soldiers into the unmapped caves to see if they can find her colleagues. What they find however is the Doctor and his traveling companions Nyssa, Adric and Tegan who have also just arrived in the TARDIS. They're near the location where the scientific expedition was attacked and Scott suspects the Doctor. There are others in the in the ...
With the Doctor cornered in the underground caves by androids, Adric ventures outside of the TARDIS and manages to destroy one of them. They get control of the situation and the Doctor manages to open the door they've located - but in doing so, he's activated a bomb. They set off to find who is behind it all and arrive on board a spaceship on its way to Earth. Unknown to them as yet is that it's all down to the Cybermen, who are determined to destroy the Earth.
The Doctor and Adric are taken prisoner aboard the freighter after they've discovered one of the crewmen dead. Captain Briggs is upset at the delay the disappearance of three crew members has caused but the Doctor denies having anything to do with that. Meanwhile, the Cyber leader activates the Cybermen they've placed in the ship's hold and sets off to take control of the ship. Unbeknown to anyone, a member of the crew is assisting the Cybermen in their quest but the Doctor finds a way to hold them off, at least temporarily.
The Cybermen are now in control of the ship and they have eliminated their human accomplice. The Doctor, Adric and several members of the crew are now their prisoners. Tegan finds herself separated from Lt.Scott and she too is soon taken prisoner and is reunited with the Doctor who realizes just what the Cybermen are going to do: use the spaceship as a flying bomb. Scott and the other troopers make a rescue attempt. Adric tries desperately desperately to stop the ship from crashing into Earth.
A British Airways Concord disappears on final approach. The Doctor, still reeling from the loss of Adric, becomes involved when his TARDIS miss-fires from its intended destination of London 1851 and appears at Heathrow airport. The Doctor follows the lost plane into the deep past only to discover he is part of an illusion. This forces him to ask the question: if there is an illusion, who is the conjurer?
The Doctor's TARDIS is stolen, and tracking it down reveals the existence of an alien named Kalid controlling events in the past. While the Doctor challenges Kalid's motives for being there, Tegan and Nyssa discover the source of Kalid's power - an even greater threat for the Doctor.
The Master commandeers the Doctor's TARDIS, leaving the stranded Time Lord time to work out the mystery of the alien sanctum he's been trying to enter. Inside lies an immense power - the entire Xeraphin race, thought long lost but distilled into a single living essence, ready for rebirth but now suffering a dual gestalt personality, good versus bad, thanks to the Master's meddling with it. As the two Xeraphin halves enter into a struggle for dominance of the whole, the Doctor works for a favorable outcome before the Master can succeed at turning them into his new ...
Incorporating the Xeraphin race into his TARDIS power system, the Master has won, but technical obstacles and a bit of sabotage from friendlies give the Doctor a bargaining chip for the lives of the hijacked and enslaved airline passengers and crews - with maybe, just maybe, some hope for the Xeraphin race as well.