The TARDIS arrives on Earth in the 26th Century in a cave system containing numerous dinosaur fossils. The Doctor's party comes under suspicion from a military force, led by Lieutenant Scott, who are investigating the disappearance of a group of palaeontologists and geologists. They are all then attacked by androids - the true culprits - under the control of the Cybermen.

The Doctor manages to deactivate a bomb intended by the Cybermen to destroy an imminent peace conference. He then follows the bomb's activating signal to an approaching space freighter, commanded by Captain Briggs, on board which it transpires that an army of Cybermen are concealed.

An attempt by Adric to thwart the Cybermen's plans result in the freighter being inadvertently sent millions of years into the past, where it explodes and causes the extinction of the dinosaurs - and also the death of Adric. The Doctor destroys the Cyber Leader by forcing into its chest unit some gold fragments from a badge previously worn by Adric.