As Voyager runs dangerously low on fuel, Seven of Nine discovers a Class-Y planet containing a high concentration of deuterium. Unfortunately, it is also called a demon planet because of its toxic atmosphere. When an attempt to transport deuterium to the ship results in an explosion, Kim and Paris volunteer to mine the fuel on the surface. They wear environmental suits, but Kim's protection is compromised when he falls into a pool of metallic compound. As Kim's oxygen rapidly depletes, Paris' suit also fails, and both men collapse before they can reach their shuttle.
With no communication from Kim and Paris, Janeway decides to land Voyager on the planet and send an away team to look for them. Chakotay and Seven don environmental suits and begin their search, quickly finding the shuttle with no one onboard. Suddenly, the ground gives way underneath Chakotay, and he barely hangs on to a ledge. As he struggles to keep from falling into the pool below, Paris appears — minus his environmental suit — and helps Seven pull Chakotay to safety.
Paris explains that he and Kim have apparently adapted to the environment, and he tells Chakotay and Seven to take off their suits as well. They refuse to take that risk and beam back to Voyager instead. As soon as they are back on the ship, Kim and Paris begin suffocating. The Doctor quickly erects a forcefield around them containing the planet's gases so they can breathe, and he surmises that fluid has entered their bloodstream that altered their physiology at the cellular level. Unless the effect can be reversed, Kim and Paris will have to be left behind in order to live.
Janeway and Torres watch in awe as the metallic compound from the fluid sample replicates Torres' thumb. Back on the surface, the away team discovers Paris and Kim's bodies. Barely alive, they are quickly beamed to sickbay, but the duplicate Kim refuses to leave, saying he feels a connection to the planet. With a large pool of the compound forming under Voyager, Janeway tries to ascend into orbit. However, the ship is restrained by an electromagnetic force.
After transporting the duplicate Kim onto Voyager, Janeway demands her ship be allowed to leave. Since he has the silver blood running through him, he tries to explain its motive for holding them there. The compound is alive, but it has never been conscious. When it entered the bodies of Paris and Kim, it experienced awareness and actually had thoughts for the first time — and now it craves more. The crew agrees to donate samples of its DNA so the planet can duplicate them and populate itself, and Voyager is allowed to resume its journey.