Above the planet, Janeway on Voyager hasn't heard back from the Away Team in a while and decides that the crew should land the ship on the surface to find them, despite the fact that in doing so they could risk not being able to lift off again. Another Away Team, this time Chakotay and Seven of Nine, is sent to explore the surface in environmental suits when they discover Tom and Harry are alive, though now somehow able to exist on the planet without environmental suits. Chakotay and Seven bring the other two on board and The Doctor discovers that they have been bio-formed to exist on the planet, with a silver liquid that has somehow been injected into them. However, because Voyager cannot replicate the environment of a Demon-class planet, that would mean that Tom and Harry would have to be left behind if they can't be restored to normal.
As Harry, Chakotay, and Seven return outside to search the area, they eventually come across...Tom and Harry still in their environmental suits. Around the same time, Voyager is being sunk into a rising pool of silver liquid. Janeway eventually learns that this liquid called "silver blood" is somehow able to duplicate whole beings from sampled DNA. Realizing that they have allowed this "silver blood" to become sentient beings that desire to live in physical bodies, Janeway allows it to duplicate the entire crew, leaving behind an entire group of humanoid beings that are capable of living on a Demon-class planet by the time Voyager leaves.
- Adam and Eve Plot: A twist on this trope, with Tom and Harry as the unwitting progenitors.
- Artistic License Physics: Deuterium is only rare in comparison to protium, the other isotype of hydrogen. It is literally one of the most abundant things in the physical universe. The later episode The Void actually made a crack about this before they find out it's actually rare in the titular void. Additionally, it can only exist as a liquid at extremely low temperatures or very high pressures.
- As You Know: Tuvok reminds Janeway that landing the ship would require a large amount of energy.
- Call-Back: Harry Kim rattles off a list of traumatic things that have happened to him, all references to previous episodes.
- Cloning Blues
- Commander Crash: A rare aversion for Chakotay, as he managed to safely land Voyager on the demon planet. Janeway and Chakotay's relieved reactions and Janeway's earlier concern of losing Chakotay if he were to take a shuttle come off as a sort of an In-Joke.
- Continuity Nod: Probably unintentional, but still a subtle bit of Fridge Brilliance — the previous episode showed that Voyager was attacked by Kyrian raiders who made off with a fair bit of tech. If this included fuel, that might help explain this ep's energy shortage.
- Death World/I Don't Like the Sound of That Place: The "Demon-class" planet.
- Double Standard: When parts of the ship are being abandoned to save power, extras in the background are carrying large luggage containers to their new lodgings but Tuvok comes down hard on Neelix for carrying a small book, a change of clothes, two pillows, and a blanket. Neelix is left with nothing but the clothes on his back.
- Extremophile Lifeforms: The Silver Blood, a deuterium-based race encountered on a Class Y planet. The episode takes its name from the fact that Class Y planets are sometimes termed "demon" worlds because they're basically Fire and Brimstone Hell (start with temperatures averaging 500 Kelvin and go downhill from there).
- Got Volunteered: Tom gets the goofy idea to set up a bicycle to power Voyager and volunteers Harry to pedal it. Harry gets back at him by volunteering him to accompany him to the Death World to get fuel. Then their duplicates get in on the act when Harry volunteers Tom to play "guinea pig" for the Doctor.
- I'm a Doctor, Not a Placeholder: "This is a Sickbay, not a dormitory."
- Insane Troll Logic: While Tom and Harry are trapped on the planet and they can't wait for them too long, as the ship is slowly running out of energy and just staying in orbit of the planet is dangerous, Chakotay suggest taking a shuttle and go look for them. Janeway dismisses this as too dangerous — so she decides to land the whole ship on the planet, which will take a big amount of energy and the ship might not even survive landing due to its weakened shields.
- My Life Flashed Before My Eyes: And Tom had just reached puberty before being duplicated.
- OOC Is Serious Business: Tom is genuinely surprised when Harry puts Tuvok in his place for being condescending. Harry replies that after everything he's been through, it's time for some Character Development. (Though the Eternal Ensign should've bucked a little harder for that promotion.)
- Quicksand Sucks: The deuterium pools act like this. One sucks Harry in, forcing Tom to rescue him; another one starts swallowing Voyager.
- Red Oni, Blue Oni: When Chakotay leads the away mission to rescue Tom and Harry, he refuses to let B'Elanna go along, saying he needs cool heads in such a dangerous environment. B'Elanna suggests Seven of Nine. Chakotay is surprised, since B'Elanna and Seven don't get along well, but she says that no one has a cooler head than Seven.
- Sci-Fi Writers Have No Sense of Scale: So Voyager is down to sublight speed, yet they find a deuterium-rich planet less than a week away at one-quarter lightspeed. If it's that close, shouldn't they have already detected it? And when their first attempt to mine the planet fails, why would Janeway consider resuming course when they wouldn't make it to anywhere else?
- Tomato in the Mirror: The Tom Paris and Harry Kim that seemed to be bio-formed to be compatible with the Demon-class planet atmosphere were revealed to be "silver blood" duplicates more than halfway through the episode.
- Torment by Annoyance: Neelix and the Doctor have a back-and-forth where the Doctor does his work in sickbay while singing opera to annoy Neelix and three other sleeping crew members out of sick bay. Neelix responds by getting his bunkmates to participate in a multi-cultural sing-a-long — to which the Doctor responds by relenting.