Demon

Voyager is critically short of deuterium and goes into power-saving “grey mode”. The only source nearby is a Demon-class planet with a toxic atmosphere and searing heat. Kim and Paris take a shuttle down and find a pool of silvery liquid rich in deuterium, but both fall victim to its effects and contact is lost. When a rescue team finds them, they are alive and breathing the poisonous air without suits. The silver liquid has altered their bodies so that they can no longer survive aboard the ship. The Doctor discovers that the substance is a sentient life form, newly aware after touching human DNA and eager to copy it. It will not let Voyager leave. Janeway strikes a deal: the crew will let it copy them in exchange for the ship's release. Kim and Paris are restored, and Voyager leaves behind a whole community of crew duplicates.