The Doctor and Dodo return to London in the present day where the Doctor becomes concerned about WOTAN, a new super-computer installed in the Post Officer Tower.

Dodo is under the control of WOTAN and attempts to lure the Doctor into a trap, while the rest of WOTAN's slaves begin construction on its robotic War Machines.

Ben tries to escape from the warehouse but is captured by Polly, who is now also under WOTAN's control, and forced to work on construction the War Machine.

The first War Machine is defeated but the Doctor only has a matter of hours to destroy WOTAN before the rest of its army is activated.

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, 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.