The TARDIS arrives at a holiday camp company with the crew worried by a prediction of giant crabs and the Doctor becomes interested in the apparent madman Medok's stories of creatures that roam the colony at night.

The Doctor is unable to convince the colonists that the Macra exist and an attempt to brainwash his companions results in Ben betraying his friends.

The Doctor, Polly and Jamie are sentenced to work in the pits and an escape attempt sees Ben's loyalties called into question again.

The Doctor and Polly manage to save Jamie from the Macra and then try to show the Pilot who is really in charge of the colony.