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 Doctor and his companions are escorted to the Sense-Sphere where they attempt to negotiate with the Sensorites for the release of the TARDIS.

With Ian having fallen victim to the Sensorites' plague, the Doctor is desperate to find a cure. But the City Administrator is still determined to rid his world of aliens.

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.

The TARDIS arrives on a planet that has been turned into a space museum but the Doctor and his friends are puzzled when they find themselves out of phase with their surroundings.

Now back on their proper time track, the Doctor and his friends must work to avoid the future they saw for themselves.

With the Doctor missing, Ian, Barbara and Vicki find themselves in danger from the Moroks while the Xerons also try to make contact with them.

While Ian tries to force Lobos to release the Doctor from the museum, Vicki joins the Xerons in trying to gain the weapons needed to start a revolution against the occupiers.