At the Federation's request, a U.S.S. Enterprise away team visits Velara III where a group of terraformers are working to transform the seemingly desolate planet into one capable of supporting life.
Tragically, a terraformer engineer is killed during their visit in a bizarre hydraulic room accident.
While investigating the mishap, Data is inexplicably attacked by the laser drilling system. He escapes injury, but upon further exploration, Data and La Forge find a microscopic inorganic life form that seems to be trying to communicate with them. They take the life form, or "microbrain" as they refer to it, back to the U.S.S. Enterprisewhere it begins to reproduce and project an energy force that compels Dr. Crusher to activate a quarantine seal around the medical lab.
Soon, the crew learns from the microbrain that some of its members were killed during the terraformer's drilling on the planet's surface, and that its attempts to contact the humans were ignored. As a result, they murdered the engineer in the hydraulic room and have declared war on the human race.
To save the U.S.S. Enterprise, Picard attempts to beam the life form back to Velara III. But the microbrain's power is too strong. Just when its energy projections threaten to destroy the ship, the crew discovers that the microbrain feeds off light. By shutting off the lights in the lab, they force the microbrain to surrender and are able to beam it back home.