After his regeneration, the Fifth Doctor (Peter Davison) is physically weak and disoriented. His companions, Adric (Matthew Waterhouse), Nyssa (Sarah Sutton), and Tegan (Janet Fielding), help him to the TARDIS. He asks them to take him to the Zero Room, where the Time-Lord healing technology is located.
Tegan and Nyssa find a terminal that tells them how to use the machine. They try to pilot the TARDIS, but it goes to a pre-set time and place—Event One, which is the Big Bang, a trap set by the Master. When they can’t find Adric—the Master has him in a trap--they manage to bring the Doctor to the console room, where he is able to jettison a quarter of the TARDIS’ mass, which propels them back into conventional time. The Zero Room, they find, was part of the jettisoned mass, so with Nyssa’s help the Doctor builds a coffin shaped Zero Cabinet. Tegan finds information on Castrovalva and directs the TARDIS there.
In the forest, Nyssa and Tegan struggle to carry the Doctor, though he levitates inside the box to help, and they become separated from him. He is captured by warriors guarding Castrovalva and taken into the town, while Nyssa and Tegan have to climb a cliff to reach the entrance. The Doctor is cared for by Shardovan the Librarian (Derek Waring) and the elderly Portreeve (Anthony Ainley) before Nyssa and Tegan arrive.
After a night’s sleep, they discover something strange about Castrovalva: no matter which way they go, they end up back in the Plaza, and a tapestry in the Doctor’s rooms changes constantly. The Doctor says they are in a Recursive Occlusion and Castovalva is not real. The Portreeve reveals that he is the Master and shows them Adric stuck in a web like a spider’s prey. The Master has been using his mathematical skills to create the town and to create the programmes in the TARDIS that brought them there.
Understanding, Shardovan the Librarian swings from the chandelier into the web and destroys it, freeing Adric and causing the town to fall apart. The Master takes off in his TARDIS as the Doctor and companions flee the town. It appears that the Master is trapped in the collapsing world. The time-travellers return to the TARDIS.
When he is disoriented after regeneration, the Doctor calls Adric “Brigadier” and “Jamie”, calls Tegan “Vicki” and “Jo”. He seems to see ice-warriors and calls for K9 and behaves like previous incarnations. He puts on his cricket uniform and pins celery to his lapel, which he will not explain until just before his next regeneration. This Doctor differs radically from Tom Baker’s outrageous personality. He is more like Tristan, actually--the character he played in All Creatures Great and Small--but Davison is a fine actor and often better than his material, which I fear was already in decline. Mimicry is one of his acting skills and his imitations of previous Doctors was rather good. In fact, he used that skill auditioning for the role.
Nyssa, traversing the forest, gradually loses the more cumbersome parts of her Princess garb—her crown, her jacket, a kind of fairy overskirt, etc.—and ends up in clothing more appropriate for adventuring, yet still stylish. Adric is trapped in a Hadron Web. He was supposed to be pale and exhausted from the ordeal, and in fact he was badly hung over that day from too much Campari the night before and was busy vomiting behind a tree when the others were talking. Tegan is competent and resourceful but complains constantly. The writers found it increasingly difficult to write for three companions, so in many adventures, one of them would be incapacitated. Louise Jameson had been offered the role of Leela again when Tom Baker was gone. I, for one, would have loved to see the gentle and sweet-natured Peter Davison travelling with a truculent knife-toting savage.
The Zero Room is cut off from the rest of the universe. As the TARDIS approaches the beginning of the universe, the cloister warning bell rings. Should it reach Event One, I will be destroyed. The real Castrovalva, in Abruzzo, was the subject of a famous lithograph by M.C. Escher. It’s too bad this story suffered from the chronic lack of special effects budget. The collapsing city was supposed to resemble M.C. Escher surrealism, but it couldn’t be done. It is only in the Twenty-First Century, with the recent Blu-Ray releases, that computer graphics caught up to the writers’ ambitions and we are treated to a decent approximation of M.C. Esher’s genius.