Arriving on a desolate and windswept Earth the Doctor starts realigning the transmat refractors while Sarah and Harry go off to explore. Harry slips and falls into a pit and Sarah runs back to the Doctor to get help. The Doctor, however, has been captured by a trio of shipwrecked Galsec space travellers who were lured to Earth by a phoney distress call. Sarah, unable to find the Doctor, makes friends with a fourth space traveller, Roth, who tells her of the alien in the rocks who is experimenting on them. Before long, Sarah and Roth are recaptured by a robot and dragged off to the rocks.

Harry finds his own way out of the pit via a series of underground tunnels and emerges close to an outcrop of rock. He watches as Sarah and Roth are presented to the alien who turns out to be a Sontaran. 

Field-Major Styre is part of the Sontaran G3 Military Assessment Survey, experimenting on the humans to try and determine their resistance to battle as Earth had now taken on a strategic importance in the Sontarans' ongoing war with the Rutans. The Doctor interrupts Styre's experiments and challenges him to unarmed combat. The Sontaran readily agrees but has not realised that Earth's unfamiliar gravity will give the agile Doctor the advantage. While the Doctor is keeping Styre occupied, Harry enters the alien's spacecraft and removes the terrulium diode bypass transformer, so that when Styre, exhausted by the fight, returns to revitalise himself, he is instead destroyed, drained of all his energy. 

As a final warning to the main Sontaran fleet, the Doctor sends a message to them, telling of their emissary's destruction and warning that without Styre's report they cannot know the strength of human resistance and so had better look elsewhere - brinkmanship at its most effective. 

The Doctor and his friends use the transmat with the intention of returning to the beacon - they have good news for Vira.

_{Synopsis from Doctor Who: The Fourth Doctor Handbook by David J. Howe, Mark Stammers and Stephen James Walker, reprinted with permission; further reproduction is not permitted. Available from Telos}