The Doctor's TARDIS travels through the 30th century but encounters a planet that's not supposed to be there. The planet is alive and swallows the TARDIS. The Third Doctor meets Harry Trant, also stranded, and they work together to get eaten by a parasitic amoeba on the planet's surface, which in turn is eaten by the planet. They reach the TARDIS in the stomach, and the Doctor materialises at the heart, telling Harry to shoot it. Hesitant, he does, and the planet screams as it dies. The Doctor tells him it needed to be killed because of its size though killing is not a pleasant duty, and takes him to his home planet.