The Cybermen have launched their deadly megatron bomb which will destroy all life on Earth unless the Doctor can deactivate the homing signal.

While the Brigadier investigates a loss of contact with Mars Probe 7, the Doctor realises signals coming from the probe are of extra-terrestrial origin.

Carrington is planning to expose the aliens on live television and call for an interplanetary war and the Brigadier must rescue the Doctor and Liz in time to stop him.

The Doctor has escaped the catastrophic destruction of a parallel world. But can he stop Stahlman before he destroys this one?