Len Skeggs is attempting to break into a house, but he is having no success. He hears a strange sound from inside the house and retreats a bit to observe.
The TARDIS materialises in the cellar of the house, and the Doctor and Peri emerge, tracking a neutrino pulse. They find a star-drive component for an alien ship in the cellar. They hear noises of distress upstairs, and make their way to the attic. In the attic, they think they find a moaning person, but it is a trap, and the door closes on them.
Meanwhile, Len sees two robed, hooded figures break into the house. He follows at a distance.
The Doctor and Peri try to break out of the attic, but then the door opens, and the Doctor realises they are being manipulated. They head back to the cellar, to find a large power cable connecting the star-drive to the TARDIS. Suddenly they realise there are two hooded figures in the cellar with them. They are priests of the Temple of Gaar on Ventros Prime, and they accuse the Doctor of stealing the Eye of Gaar, a gemstone that is connected to the star-drive. The Doctor tries to explain that they were framed, but the priests shoot blue fire at the Doctor and Peri, paralysing them and causing them great pain. Suddenly an old man, the owner of the house, destroys the priests. He explains that he stole the Eye of Gaar, and used the Doctor and Peri as a diversion so that he could repair his ship and escape the priests. He tells them the paralysis will wear off, then takes the gem in its casing and leaves. However, unknown to him, Len is lurking nearby and hits the old man with his jemmy. He is shocked to discover that the "man" is not flesh but metal, and while the man recovers, Len takes the gem casing and runs off.
The man chases after him, and soon the Doctor and Peri recover and follow. They discover the "man" nearby, dying. He is actually a worm-like creature encased in a life support system. He neglected repairing his life support unit in order to chase Len, and being from Ventros Prime, he bubbles and bursts into flame. The priests' spaceship self-destructs soon afterwards.
The Doctor tries to find Len, but Len has broken the gem's casing and the Doctor cannot track him. Len grabs the gem, but it burns his hand with cold, and turns into gas, killing him.
- Ventrosians come from a frigid planet named Ventros Prime and die on exposure to Earth.
- The Doctor assures Peri that there are no blood-sucking octopuses on Mars, just "seven-foot-tall humanoid warrior lizards".
- The Doctor has Peri call UNIT for a clean-up squad.
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