The Doctor and Sarah are held responsible for the deaths on Zeta Minor.
One of the guards, O’Hara, fires at the creature, but it grabs him and he vanishes, screaming, only for his body to reappear, drained and mummified like the others. The Doctor is not sure what the creature is, but tells Sarah he has a very unpleasant theory. Ponti, who heard O’Hara’s screams, informs Salamar that the base in under attack and they find the two time travellers missing. The Morestrans fire at them as they run away into the jungle.
Dawn breaks on Zeta Minor and the creature does not seem to like daylight. Vishinsky launches the oculoid tracker, a flying drone with a camera eye, to search the jungle for the fugitives. Meanwhile, Sorenson confirms that O’Hara died the same way as the other members of his expedition, through total dehydration — a kind of rapid freeze drying. Sorenson brushes off the deaths as irrelevant. His mission to Zeta Minor was to find a new source of energy to replace Morestra’s dying sun and he has succeeded. Sorenson demands that his mineral samples be taken aboard and they leave the planet immediately. However, Salamar says that alien forces must be found and eliminated.
The tracker finds the Doctor and Sarah near the pit in Sector 5, a dark pool with no reflection. A party arrives, led by Ponti, but as they search the two roughly, Ponti falls into the pit with a cry. The Doctor warns the rest back, telling them they are tampering with the balance of nature on the planet and it may already be too late. At the base, Sorenson gets De Haan to help load his canisters of refined ore onto the ship. Sorensen excitedly notes that six pounds of ore could produce heat equal to the output of their sun for three centuries; full scale exploitation of Zeta Minor would provide perpetual energy.
The Doctor implores Salamar to listen to him: Zeta Minor is the boundary between the known universe and one of anti-matter. In coming here, they have crossed that boundary. He warns Sorenson that if he takes those samples, they will never leave this planet, but Salamar orders the Doctor and Sarah taken to the quarantine area. Sarah suggests they simply leave in the TARDIS, but the Doctor says that the Morestrans are endangering the universe as well as themselves. He opens a canister of refined ore and takes a few crystals, placing them in an old toffee tin to test a theory.
The ship tries to take off, but the systems do not respond properly. The creature attacks the ship. The force fields are raised, but there is not enough power to repel its pure energy form. Several Morestran crewmen rush out to fire at the creature, but to no avail. It drains them one by one. The Doctor tells them to link the force field to the atomic accelerator. Salamar hesitates, but Vishinsky says they have to try and he reluctantly gives the order. This seems to work; the creature is driven back and vanishes.
The Doctor tries to reason with Sorenson; as long as the ore is aboard, the creature will return, and they will be trapped here. If they jettison the canisters and make clear their intention to leave empty-handed, they will be allowed to take off. The Doctor offers to communicate that intention to the creature. Salamar agrees to let him go alone, but launches the tracker to observe him. When the Doctor reaches the pit, the creature rises, engulfing the Time Lord. He falls into the pool…