The TARDIS is adrift, the crew turning on each other. Only by working together can they find their way out before they are destroyed.

The 28th century. The TARDIS materialises on a spaceship orbiting a planet known as the Sense-Sphere, whose natives, the Sensorites, have trapped the ship's human crew in a state of paralysis.

The Doctor and his companions are on board an Earth ship trapped in orbit around the Sense-Sphere when it is boarded by the Sensorites, who steal the lock mechanism from the TARDIS.

The City Administrator is determined to destroy the Doctor and his friends and his paranoia prevents the Doctor's cure from being distributed.

While the City Administrator's schemes reach their climax, the Doctor and Ian venture into the tunnels under the city to discover the real cause of the Sensorite plague.