To escape from the volcanic eruption on Dulkis, the Second Doctor uses an emergency unit. It moves the TARDIS out of normal time and space. The travellers find themselves in an endless void where they are menaced by white robots. Having regained the safety of the TARDIS, they believe they have escaped — until the ship explodes. They find themselves in a land of fiction, where they are hunted by life-size clockwork soldiers and encounter characters like Rapunzel, the Karkus, and Swift's Lemuel Gulliver.

The Doctor answers a summons and finally returns to his homeworld, Gallifrey, seat of the Time Lords. However, when the President of the High Council is assassinated, he becomes the prime suspect, while an old enemy lurks in the shadows, pulling the strings.

Intercepted while travelling between Earth and the Ark, the Fourth Doctor and his companions are transported to the planet Skaro on a mission for the Time Lords — to prevent the creation of the Daleks.

Arriving in London at the end of the 19th Century, the Doctor and Leela make friends with a police pathologist, Professor Litefoot, and learn that hairs taken from the clothing of a dead body found floating in the Thames seem to have originated from a very large rat. The Doctor's investigations take him first to the sewers, where there are indeed giant rats on the loose, and then to the Palace Theatre, where a stage magician, Li H'sen Chang, is procuring young girls for his master, the ancient Chinese god Weng-Chiang. Weng-Chiang is in fact Magnus Greel, a war criminal from the 51st Century. The journey back through time has disrupted his molecular structure and he now needs to feed on the life force of others - hence his use for the young girls. He has come to London to retrieve his lost time cabinet, which is in the possession of Litefoot. Infiltrating Litefoot's home with Chang's ventriloquist doll Mr Sin - a computerised homonculus with the brain of a pig - he retrieves the cabinet and prepares to travel back to his own time. The Doctor, aided by Leela, Litefoot and Henry Gordon Jago, the proprietor of the Theatre, tracks him down to his lair and traps him before he can escape. Greel falls into his life force extraction machine and disintegrates. The Doctor is then attacked by Mr Sin but manages to disconnect its circuitry, rendering it inanimate.

The TARDIS materialises in a hostile jungle on the planet Spiridon. Jo sets out alone to find help for the Doctor, who has fallen into a coma. She meets a party of Thals and is left in hiding aboard their crashed spaceship while they go to the Doctor's aid. The Time Lord, now recovered, learns of their mission to destroy a party of Daleks sent here to discover the native Spiridons' secret of invisibility. Another Thal spaceship crash-lands in the jungle, and the survivors bring news that somewhere on Spiridon there is an army of ten thousand Daleks. Jo meanwhile meets a friendly Spiridon named Wester, who cures a deadly fungus disease that she has contracted. It transpires that the Daleks' army is frozen in suspended animation in a cavern below their base. The Doctor, with the help of the Thals, explodes a bomb in the cavern wall and thereby causes one of the planet's natural ice volcanoes to erupt, entombing the army in a torrent of liquid ice. The newly-arrived Dalek Supreme and his aides are left stranded on Spiridon as the Thals steal their ship and the Doctor and Jo depart in the TARDIS.