Change was the order of the day in the twenty-first season of Doctor Who, during which Peter Davison handed the role off to Colin Baker, who debuted with a complete story rather than just a brief cameo. Companions Tegan and Turlough both left as well, replaced by a buxom American called Peri. The Daleks and their vile creator also returned to renew their enmity with the Doctor, as did the Silurians and the Sea Devils, the aquatic menaces from the third Doctor’s era. Naturally, the Master put in an appearance, too.
From “Warriors of the Deep”
On the way to show Tegan Jovanka a little bit of Earth’s future, the TARDIS materializes further into the future than the Doctor intended, due to “a slight hiccup in our time zones.” The three travelers find themselves on an undersea military base in 2084 which comes under attack by Silurian forces. Regretting the outcome of their last encounter, in 1970, the Doctor tries to once again broker peace between the human and Silurian armies, without success. In the end, only the Doctor, Tegan, and Turlough are left alive.
Preston: What happened?
The Doctor: Hexachromite. It does that to all reptile life.
Preston: Then use it on the invaders!
The Doctor: And kill them?
Preston: Why not? They’re about to start a war that will kill everyone on Earth!
The Doctor: I sometimes wonder why I like the people of this miserable planet so much! The Silurians and Sea Devils are noble races! They have skills and talents you pathetic humans can only dream about!
From “The Awakening”
The Doctor and Turlough are still working to eliminate the time distortion from the new main console when the TARDIS materializes in England in 1984 for Tegan to visit her grandfather, who happens to be mixed up in the attempts of a lunatic to access an evil alien psychic force. When the force projects itself into the TARDIS, the Doctor attempts to use the signal conversion unit to disrupt its power.
While the Doctor is in a manic tidying fit, an alarm on the console sounds, the display reading “time parameters exceeded.”
Turlough: Doctor, something’s happening to the controls!
The Doctor: Ah, we must be on the outer limits. The TARDIS has drifted too far into the future. We’ll just slip into hover mode a while.
Tegan: We’re in the Veruna system, wherever that is.
The Doctor: I had no idea we were so far out. Veruna -- that’s irony for you.
Tegan: What is?
The Doctor: Veruna is where one of the last surviving groups of mankind took shelter when the Great -- yes, well, I suppose you’ve got all that to look forward to, haven’t you?
Tegan: The Great what, Doctor?
The Doctor: All civilisations have their ups and downs...
Turlough: “Fleeing from the imminence of a catastrophic collision with the sun, a group of refugees from the doomed planet Earth -- ”
The Doctor: Yes, that’s enough, Turlough.
Tegan: You mean some of the last surviving humans are on this planet?
The Doctor: Yes.
Tegan: Can we land? Can we visit them?
The Doctor: Laws of time.
Tegan: Since when has that ever stopped you?
The Doctor: Now, we mustn’t interfere. The colony’s too new, one generation at the most. The future hangs in the balance.
As it happens, the TARDIS is forced down, and the Doctor very reluctantly helps the colonists, though he keeps insisting he is there unofficially and is not allowed to give them much assistance or make any material difference to the development of the colony, which has fallen prey to marauding Tractators. During a meteorite shower, the TARDIS is apparently disintegrated, leaving only a pile of rubble and the hat stand. Actually, its sections are scattered about Frontios’ subterranean caverns until pulled back together by the gravitational energy of the Tractator leader.
Plantagenet: Frontios is honored, Doctor. But surely you’ll stay awhile longer and enjoy some of the new colony we’re building.
The Doctor: Oh, no, no. Far too much repair work of my own to be done. Besides, time and the time laws don’t permit it. There’s an etiquette about these things which we’ve rather overlooked, I’m afraid.
However, after dematerialization, the TARDIS goes haywire.
From “Resurrection of the Daleks”
The TARDIS has, in fact, collided with a time corridor, which pulls them back to 1984 England. The time corridor is operated by the Daleks, trying to liberate Davros from imprisonment by the future Earth government. Dalek agent Commander Lytton informs Davros that the Daleks lost their war with the robotic Movellans and sustained heavy casualties due to germ warfare. The Daleks release Davros from ninety years in suspended animation in search of a cure for the Movellan virus. Realizing that Davros is tying to take control of his creations, the Supreme Dalek orders him executed, dividing the Daleks into two factions. Davros releases the samples of Movellan virus in retaliation, only to find he also is susceptible to its effects. The Dalek spaceship and the prison satellite are both destroyed. Lytton is left stranded on Earth after abandoning the fight. Sickened by the carnage, Tegan decides to part company with the Doctor and remain on Earth.
From “Planet of Fire”
Having been hidden away aboard the TARDIS all season, Kamelion turns up again, still under the control of the Master. He takes control of the ship, sending it first briefly back to Earth to retrieve a certain artifact, then on to the planet Sarn. The Master is trying to recover from an accident with his tissue-compression eliminator which has left him miniaturized. Like Earth, Sarn is a world to which exiles from Turlough’s home planet of Trion are sent. Turlough discovers that his father’s ship crashed there and the Sarn’s “Chosen One” is his brother, who was an infant found in the wreckage. The planet crumbling, Turlough is forced to contact Trion and request a rescue ship. When it arrives, however, he learns the new government has released all political prisoners and he is a free man. Turlough elects to part company with the Doctor and return to Trion with his brother. In foiling the Master’s scheme, the Doctor is forced to destroy Kamelion, and the Master appears to be burned to death in the volcanic control room.
During their brief time on Earth, Turlough rescues a drowning girl and takes her aboard the TARDIS. She is Perpugillian Brown, an American college student vacationing with her mother and step-father in Europe. Her step-father, Professor Howard Foster, stranded Peri alone aboard their boat to prevent her from going off to Morocco with friends. Peri tries to swim ashore, and is floundering when Turlough saves her. The TARDIS is already on its way to Sarn before the Doctor realizes she’s aboard. At the conclusion of the adventure, Peri asks permission to travel with the Doctor until her summer vacation is over. Rather than go on alone, the Doctor accepts the arrangement. Of course, Peri never makes it back to Earth, but ends up living on Krontep with King Yrcanos after apparently being abandoned by the Doctor. Presumably, Professor Foster would believe Peri drowned trying to swim ashore, although no body would ever be found. He would undoubtedly blame himself for her death. Her American passport and collection of alien flora are presumably still somewhere aboard the TARDIS.
From “The Caves of Androzani”
Sharaz Jek: I have missed so much of life these last lonely years. But your arrival has changed all that. We shall become the best of companions!
The Doctor: What do you say, Peri? We can go on nature walks, have picnics, and jolly evenings round the campfire.
Sharaz Jek: Don’t mock me, Doctor! Beauty I must have, but you are dispensable.
The Doctor: Thank you.
Sharaz Jek: You have the mouth of a prattling jackanapes... but your eyes, they tell a different story.
Lethal poisoning by spectrox toxæmia induces a regeneration in the Doctor as he lies on the floor of the TARDIS. He says it “feels different this time” before beginning to hallucinate. Images of his recent companions give way to the gloating face of the Master, which perhaps gives the Doctor the will to survive. It will prove to be a troubled regeneration, however.
From “The Twin Dilemma”
Burning out the spectrox toxæmia has interfered with the Doctor’s regeneration, leaving him susceptible to periodic fits of mental imbalance, such as adopting a ridiculous costume, accusing Peri of being a spy, and then becoming convinced he must become a hermit. Seeking a proper hermitage, the Doctor stumbles into a kidnapping scheme involving his old friend and mentor, Azmael, whom he last saw during his fourth incarnation. Azmael is a Time Lord in his final incarnation who apparently left Gallifrey to rule the planet Joconda. Azmael dies in the Doctor’s arms while saving his planet from destruction. Back in the TARDIS, the Doctor assures Peri that his mental state has completely stabilized.