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. 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 weeps over his body. 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 different "values and customs", and that: "No matter what else happens, I am the Doctor... whether you like it or not!" The Doctor is unusually violent at the start of this episode, even attempting to strangle Peri. The intention was to create a Doctor that was initially unlikeable, but would gradually reveal a kind-hearted soul (glimpsed in "Revelation of the Daleks"). This was also intended to be a contrast to the instantly likeable Tom Baker and Peter Davison Doctors. However, in later interviews, director Peter Moffatt said that the original idea was merely to have the Doctor in a much more energetic state than he was during the Fifth Doctor's début story "Castrovalva". Colin Baker said during a 2003 documentary celebrating the series' 40th anniversary that "the idea was that over the many, many years I would be playing the part, the outer layers would gradually peel away, revealing the kind-hearted soul." At least one aspect of Steven's original script featured the Jaconda and Gastropods being dropped totally early in the fourth episode without resolution to the plot, with the final battle taking place in another dimension against a being called Azlan who was controlling Mestor all along. The cat badge worn by the Sixth Doctor on his lapel for this story was handmade and painted by Suzie Trevor, and purchased for the programme from a specialist badge shop in central London.