The TARDIS, an interplanetary time machine that on the outside resembles a police call box, lands Doctor Who and his companions, the chirpy Dodo and forthright Steven, on a spacecraft called the Ark. They learn that Earth is about to plunge into the sun. The Ark contains all human, plant, and animal life and is bound for the planet Refusis. But Dodo has a cold, against which the Ark's Guardians and the slave race called Monoids have no immunity.
The Doctor is able to overcome the Guardians' fears and suspicions and finds an antidote. The TARDIS is allowed to take off, but it returns, seemingly seconds later. Actually 700 years have passed. The time-traveling trio is stunned to discover that a mutant strain of the fever has led to an evolutionary shift--the Guardians are now the slaves and the Monoids rule.
The Ark is disarmingly stagey (some of the actors step on each other's lines and others stumble over their dialogue), but it is a precious relic from television's longest-running sci-fi series, and one for which its devoted fans will gladly make time. --Donald Liebenson