James Sands and his team have been surveying Planet Z02-80535 for six months. Their job is to search for sentient life on the planet for Trans-Planetary Mining. Mining of the planet will begin in 24 hours, as the survey team hasn't found any sentient life thus far.
They have, however, been experiencing mechanical faults, accompanied by the escaping of their test subjects — natives of the planet. Sands thinks it is sabotage, either by TPM or a rival firm. Scientist Jessica Copeau thinks it is mechanical failure.
Michael Kelin and Erich Strauss are having their usual argument over how valuable the native life on the planet is, when Copeau enters, holding two strangers at gunpoint. The strangers introduce themselves as the Doctor and Sarah Jane. They landed outside the building and were caught jimmying the airlocks.
Sands suspects them of the sabotage, but Copeau starts to believe that the Doctor can help. Sands gives him a chance to investigate. The Doctor examines the laboratories, where the test subjects have been held. What he sees leads him outside. The others accompany him. He shows them a large, slug-like creature, and tells them it is a Builder, the sentient life on this planet. Builders have the ability to select their offspring's genetic capabilities, and the specimens collected by the team are sentient. They caused the sabotage and escaped.
Suddenly a large noise is heard. The mining operations have begun, twelve hours early. The survey team is devastated that they are responsible for the destruction of a sentient species. However, the Builders start fighting back. In the confusion, the Doctor and Sarah head back to the TARDIS. The survey team experiences a brief moment of elation that the Builders won't be destroyed, right before the Builders destroy the structure the team is in.
Back in the TARDIS, Sarah wonders why the Doctor brought her here. He tells her he always wanted to know how the Builder Wars started.
- Copeau uses a laser torch as an improvised gun.
- Kelin and Strauss drink coffee.
- The survey team use spacesuits on the planet's surface.
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