After his regeneration, the Doctor starts behaving erratically. He goes to the wardrobe and looking for a new outfit and finds a glaring, mismatched, brightly coloured coat to which he immediately takes a shine. Peri tells him that he could not go outside wearing such an awful garb, to which the Doctor takes offence.
Two twins, Romulus and Remus Sylveste, receive a visitation from a mysterious old man called Professor Edgeworth. They question how he managed to get inside their house; he tells them he will return when their father is there, then proceeds to abduct them and the trio disappear. They arrive on a spacecraft in deep space. Edgeworth then communicates with his superior, a slug-like creature called Mestor, who instructs Edgeworth to take the twins to Titan 3.
In the console room, the Doctor has a funny turn, quoting a poem about a Peri — a good and beautiful fairy in Persian mythology, but one which used to be evil. The Doctor accuses her of being evil, and of being an alien spy before rushing toward her and throttling her. He catches a sight of his own manic face in a mirror and collapses in a heap, releasing Peri. When she tells him that he tried to kill her, he initially denies he could be capable of such an act, but seeing how terrified of him she is, decides he must become a hermit on the desolate asteroid Titan 3.
The twins' father contacts the authorities; he found Zanium in their room — a sure sign of intergalactic kidnap. A Commander Lang begins the pursuit and soon finds a suspicious ship previously reported missing. He tries to contact it, but it enters warp drive — something that class of ship is not designed to do.
On Titan 3, as the Doctor contemplates a thousand years of solitude and Peri expresses her disapproval, they hear the crash landing of a craft. Examining its wreckage, they find the concussed body of Commander Lang. They take him back to the TARDIS where he reveals his whole squadron has been destroyed. Believing the Doctor to be responsible, he points his gun at the Doctor and threatens to kill him…
Peri pleads with Lang, telling him that the Doctor had in fact saved him, but he faints away. The Doctor is not keen to treat Lang, more concerned for his own life, but eventually agrees to Peri's persuasion.
Edgeworth argues with Romulus and Remus, making them do Mestor's work. He scolds them for setting up a distress signal, so they are not allowed to use electronic equipment to solve the equations they have been set. An image of Mestor appears and gives the twins a more blunt threat — work for him or have their minds destroyed.
On the TARDIS scanner, the Doctor and Peri see a building — something which has no place on an uninhabited asteroid. Leaving Lang behind, they find a tunnel which may lead to the building, but on exploring find two aliens wielding guns. The Doctor cowers in fear and pleads with them not to shoot him. They are led off and are brought before Edgeworth. The Doctor claims to be a pilgrim to Titan 3, but Noma, one of the aliens, says they are spies and should be shot. The Doctor suddenly recognises Edgeworth as an old friend - Azmael, master of Jaconda, whom he last saw two incarnations ago. When the Doctor sees Romulus and Remus and discovers it is Azmael who has abducted them, he is disgusted. Azmael teleports away with the twins and the aliens, leaving the Doctor and Peri locked in the building. The Doctor starts to break the lock's combination, but Peri discovers Noma has set the base to self-destruct. The Doctor improvises a solution to teleport them back to the TARDIS. Peri makes a successful return, but the Doctor has not appeared when she sees the base explode on the scanner…
A glimpse of the Doctor is seen appearing in the TARDIS; he was delayed returning because he was using Peri's watch to synchronise their arrival, but the watch had stopped. The Doctor is surprised at Peri's compassion when she thought he had died.
On Jaconda, Mestor is seen putting one of the bird-like Jacondans to death for a petty offence of stealing a few vegetables. Soon, the TARDIS arrives, but instead of the expected beautiful planet the Doctor is expecting, he, Peri and Lang find a desolate wasteland covered with giant Gastropod trails. The Doctor is reluctant to go to the palace, scared for his own life, but is persuaded to take Lang there in the TARDIS. In the palace corridors they see murals depicting Jaconda's history, depicting the slugs of myth - but it appears that they are now all too real. After avoiding Gastropods, Lang gets stuck in their slime trail.
Azmael takes the twins to his laboratory and shows them a store room full of Gastropod eggs. Mestor arrives and tries to persuade them that his aims are benevolent. Azmael begs him to stop reading his thoughts and stop Noma watching his every move. He agrees and leaves. Azmael explains to the Twins that Mestor usurped him as leader of Jaconda and outlines a plan to draw two outlying planets into the same orbit as Jaconda. The Twins' genius is required to stabilise those planets in their new orbit. The Doctor, leaving Peri and Lang behind, finds Azmael's lab. In a manic fit of pique, he attacks Azmael, but is restrained by a Jacondan and the Twins. The Doctor apologises to Azmael but demands to know what is going on.
Meanwhile, Peri is captured by Jacondan guards and brought before Mestor. When Lang escapes to Azmael's lab and informs them what has happened, the Doctor finally shows compassion for her when he thinks she might die…
Mestor refrains from killing Peri immediately, finding her appearance pleasing. Jacondan guards arrive in Azmael's lab and seize the Doctor. The Doctor tells Mestor that he ought to allow him to assist with the dangerous operation of moving the planets, as a single mistake could blow a hole in that corner of the universe. Back the laboratory, Azmael informs the Doctor the details of the plan to bring the planets into the same orbit — they will be placed in different time zones using time travel technology that Mestor stole from Azmael. The Doctor realises that, as the other planets are smaller than Jaconda, bringing them closer to Jaconda's sun will lead to catastrophe. The Doctor enters the egg storeroom, and is disturbed that they have no nutritional mucus. He tries to cut one open with a laser cutter; the shell is impenetrable, but the egg reacts slightly to the heat. The Doctor realises they have been designed to withstand the heat of an exploding sun — the explosion of the Jacondan sun will scatter the eggs throughout the universe. When they hatch, the Gastropods will conquer the universe.
The one remaining Jacondan in the lab collapses dead, his mind burnt out. Mestor had been using him as a monitor, and knows the full details of what has been discussed. Peri, Lang and the Twins return to the TARDIS, whilst the Doctor and Azmael go to confront Mestor. When Mestor refuses to abandon his plans, the Doctor hurls a vial of acid taken from the lab at him, but a force field protects Mestor from any harm. Mestor threatens to possess the Doctor's mind and body, and demonstrates by taking control of Azmael's body. Azmael tells him to destroy Mestor's body before he can return to it, which he does with a further vial. Then Azmael, in his last regeneration, forces himself to regenerate — killing himself — and in doing so destroys Mestor. Dying, Azmael says he has no regrets and that one of his fondest memories was a time spent with the Doctor by a fountain.
The Doctor and Peri return to the TARDIS; Lang decides to stay behind on Jaconda to assist with their rebuilding. When Peri tells the Doctor off for being rude, he reminds her that he is an alien, with alien sensibilities: "I am the Doctor… whether you like it or not!"
Doctor Colin Baker (Sixth Doctor)
Companion Nicola Bryant (Peri Brown)
Maurice Denham — Edgeworth
Edwin Richfield — Mestor
Kevin McNally — Hugo Lang
Dennis Chinnery — Sylvest
Gavin Conrad — Romulus
Andrew Conrad — Remus
Seymour Green — Chamberlain
Oliver Smith — Drak
Barry Stanton — Noma
Dione Inman — Elena
Helen Blatch — Fabian
Roger Nott — Prisoner
John Wilson — Jacondan Guard
Writer Anthony Steven
Director Peter Moffatt
Script editor Eric Saward
Producer John Nathan-Turner
Executive producer(s) None
Production code 6S
Series Season 21
Length 4 episodes, 25 minutes each
Originally broadcast March 22–March 30, 1984