The time-travellers arrive at an Earth Colony on an unnamed planet in the far future. They meet a half-crazy colonist named Medok (Terrance Lodge) who is arrested by the chief of police (Gertan Klauber) taking orders from a mysterious Controller (Graham Leaman) who appears on a television screen. There is a festival going on in the colony that seems phoney and wrong to the Doctor. Medok is accused of Deviation for being unjoyful. He warns of horrible clawed creatures in the night but is ignored. The Doctor helps him escape and is arrested himself. He and his companions are sent to hard labour in the mines, which are filled with dangerous gasses. The Doctor slips out and finds Medok, and together they track down the giant crab-like Macra.
In the second and third episodes, various characters escape from the mines and are re-captured, are hypnotized into obeying the Controller, shake off the spell, and are hypnotized again. The Doctor gradually pieces together the whole picture. The dangerous gasses in the mines, it turns out, are essential for the Macra to live. In fact, the entire colony exists to provide gas for the Macra, and the colonists are brainwashed into serving the creatures. Finally, a mixture of explosive gasses meant to kill the time-travellers are diverted and used to blow up the Macra. The travellers leave the colony, now the happy place it pretended to be.
The serial was praised by critics for the acting of the guest-cast, who created interesting and believable characters. Some thought the Macra themselves—just another monster—let the story down, but some praised the construction of the crab-like creatures. The entire story was lost, leaving a few fragments and still pictures, but was reproduced in animation in 2019. The animated Macra were more believable and threatening. The creatures re-appeared in the “Gridlock” episode of the David Tennant years, the smog-breathing monsters living below the 20-year-long traffic jam in New New New New New New New York.