The TARDIS crew arrives on Earth in the 26th Century in a cave system where a military force is investigating the disappearance of a palaentological expedition.

With her crew vanishing mysteriously, Captain Briggs assumes the Doctor is the guilty party, but the traitor is closer to home… as are the Cybermen.

The Cybermen are released from their silos and are on the march, attacking Briggs's crew and ensuring that the freighter remains on course for Earth. But why?

The Doctor realises the freighter is now a huge bomb and sends it back through time where it can explode millions of years ago... but Adric is still aboard...