The Sixth Doctor (Colin Baker) behaves erratically after his regeneration. All the Doctors try on their new outfits, but his is a glaring mismatch of colors which he sees as exquisite. He accuses Peri of being evil and tries to strangle her, until he sees himself in a mirror and is shocked. He decides to become a hermit on the asteroid Titan 3.
The mathematical genius twins Romulus and Remus Sylvestre (Gavin and Andrew Conrad) are abducted by a mysterious Professor Edgeworth (Maurice Denham) and taken to a spacecraft, where Edgeworth speaks to his superior, a slug-like creature called Mestor (Edwin Richfield), who tells him to take the twins to Titan 3. The twins’ father finds zanium in their room, which as everybody knows is a sure sign of intergalactic kidnapping, and he contacts Commander Lang (Kevin McNally) who finds a ship that had been reported missing. To escape him, it goes into warp drive, which it is not supposed to do.
On Titan 3, the Doctor and Peri hear a crash. In the spaceship’s wreckage they find the unconscious body of Commander Lang. They take him into the TARDIS, where he tells them his whole squadron is dead. Naturally, he blames the Doctor for this and pulls a gun on him. Peri pleads for his life, telling him the Doctor saved him. Lang passes out again. Edgeworth argues with Romulus and Remus and forces them to do Mestor’s mathematical work. Mestor assures them he will destroy their brilliant minds if they do not.
The Doctor and Peri find a building on the supposedly uninhabited rock. They investigate and find armed aliens. The Doctor begs them to shoot Peri instead of him. They are dragged off, accused of being spies, and sentenced to be shot. But the Doctor recognizes Edgeworth as his old Time Lord friend Azmael, Master of Jaconda. When he learns that Azmael has abducted the twins, he is disgusted. Azmael teleports away with the boys, leaving the Doctor and Peri to die when the base self-destructs. The Doctor finds a way to teleport separately back to the TARDIS. When he appears a little late, he is surprised to find that Peri was worried about him.
On Jaconda, Mestor puts one of the bird-like Jacondans to death. The TARDIS arrives in the desolate Jacondan wasteland. The Doctor is frightened but takes Lang into the palace. Azmael takes the twins to his lab and shows them gastropod eggs. It appears the twins’ math skills are needed to stabilize two planets into the Jacondan orbit. In Azmael’s lab, the Doctor attacks Azmael, but is restrained by the twins.
Peri is captured by Jacondan guards and brought to Mestor. He finds her too pleasing to kill right now. The Doctor is seized as well and Azmael informs the Doctor of his plans, so that he may help. The Doctor realizes the plan will fail catastrophically. Examining the gastropod eggs, he understands that they are designed to withstand the heat of an exploding sun and will be scattered throughout the universe, allowing them to hatch and conquer all.
A Jacondan named Drak (Oliver Smith) dies in shock with the knowledge of what he has helped to bring about. Peri, Lang, and the twins return to the TARDIS, while the Doctor and Azmael confront Mestor. Mestor threatens to take over the Doctor’s body, and to demonstrate his power, takes control of Azmael’s body. Azmael tells the Doctor to quickly destroy Mester’s body, which he does with acid. Azmael, now in his last regeneration, dies, killing Mestor in his body. The Doctor weeps. Lang decides to remain on Jaconda to assist with rebuilding. The Doctor and Peri leave in the TARDIS, arguing.
To be cast as the Doctor, an actor needs three attributes. One, he must be a good actor because much of what he says is nonsense. Two, he must be able to work under BBC conditions of time and budget. Three, he must be completely different from the previous Doctor. Since Peter Davison was so likable, Colin Baker had to be a bit of a bastard. He was supposed to mellow over time and become nice. Unfortunately, this Doctor was dropped after little more than a year and never had the time to develop a winning personality. He became less violent but remained loud and arrogant to the end.
Before trying to strangle Peri, the Doctor quotes the poem Lalla Rookh by Thomas More; “One morn a Peri at the Gate of Eden stood disconsolate.” A Peri is a good and beautiful fairy in Persian mythology who was born evil, suggesting to the Doctor’s muddled brain that Peri was too. Following the Caves of Androzani, frequently called the best Doctor Who story, this one has always been widely considered the worst. We are used to the Doctor acting strangely after regenerating, but this was a bit beyond the Pale. Later producer Russell T. Davies said it was the beginning of the end of Doctor Who.