With their deuterium supplies dwindling, Captain Janeway orders the crew of Voyager to reduce power consumption to minimal levels, traveling at impulse power and confining the crew to a few decks. The crew detects signs of deuterium on a nearby planet, but it is quickly identified as a "demon"-class planet, hostile to living creatures due to its high surface temperature and toxic atmosphere. With no way to remotely extract the deuterium due to communication interference from the atmosphere, Harry Kim volunteers himself and Tom Paris to take a shuttlecraft to the surface using modified shield technology and extract the essential materials themselves. As they track down the deuterium deposits on the surface, they discover a metallic-like liquid that maintains a cool temperature despite the atmosphere. Soon their environmental suits fail, and they collapse.
When Paris and Kim fail to return to Voyager after some time, Janeway orders the ship to land on the surface to send out teams to find them. Chakotay and Seven discover the two alive, but out of the suits and apparently able to withstand the harsh conditions. With a supply of deuterium to continue their journey home, they return to Voyager but Kim and Paris start to suffocate once aboard until the Doctor can contain a supply of the planet's atmosphere. The Doctor discovers both men have a large quantity of the metallic liquid in their blood, and explains that because the planet's atmosphere is impossible to recreate, should Voyager leave, they would have to leave the two behind. Soon, a pool of the liquid forms beneath the ship, and before they can lift off, the ship is partially sunk, trapping them. Torres discovers the liquid has the ability to take on the form of organic material.
Chakotay, Seven and Kim return to the planet, and eventually discover the bodies of Kim and Paris, still within the environmental suits running on emergency backup power. The two are safely returned to Voyager, and the Doctor realizes that the other Kim and Paris are actually clones created by the metallic liquid, using the DNA from the human crewmembers to construct the bodies. When they try to return the two clones to the surface, the pool under the ship reacts, causing it to sink more. Janeway is able to talk to the Kim clone, and reaches out to this new intelligence, a "silver blood", who explains that it has just discovered life and wants to keep the Voyager crew around for companionship. Janeway proposes to the silver blood that it can clone any member of the crew that volunteers, allowing it to create duplicates of the crew for itself, in exchange for releasing the ship. The creature agrees, and soon Voyager lifts off, leaving numerous crew clones on the planet.