Voyager lands on a scorching Demon-class planet to obtain deuterium and finds a silvery liquid that copies Kim and Paris.
Running short of deuterium, Voyager finds a supply on a Demon-class planet whose heat and toxic atmosphere are deadly to humanoids. Harry Kim and Tom Paris go down in environmental suits and encounter a strange silvery liquid. When their suits fail they are found apparently able to breathe the planet's air, but they cannot survive aboard the ship.
The crew then finds the real Kim and Paris still on the surface and realizes the others are duplicates formed by the liquid, which has become sentient through contact with them. When the substance traps the landed Voyager, Janeway agrees to let it copy any crew members who volunteer, and the ship departs leaving the duplicates behind. Their fate is followed up in Course: Oblivion.