They set out, and while the air was initially mostly helium, necessitating EV suits, it changes to oxygen and nitrogen. Even though they still don't detect any lifeforms, they begin to suspect that the blue lights are sentient. One of them then flies into Trip's head and flies out again, having turned golden. Trip seems a bit out-of-it, until the golden wisp flies back into his head and the blue wisp flies out. He claims he saw the away team from above and himself swimming with his ex-girlfriend Lisa.
Archer and Malcolm take Trip to sickbay, but Phlox finds nothing wrong with him, so Archer tells Trip to get to work fixing the engines and Malcolm to get to work fixing the weapons systems. Archer then goes over the situation with T'Pol in his quarters, where they discuss whether or not the aliens are hostile. T'Pol also reveals that Hoshi has been trying, but failing, to communicate with them.
Trip is working in engineering, when a wisp possesses him again. He leaves, Michael Rostov tells Archer about it, and the two find the possessed Trip eating in the mess hall. Archer confronts him, and the creature admits to possessing Trip but promises to bring Trip back, claiming that its people evolved from corporeal beings and are curious about it. At Archer's request, it frees Enterprise and sends the golden wisp back, which returns Trip to normal. Trip claims that the aliens showed him more memories and offered to do the same to the whole crew, prompting Archer to return him to sickbay.
Archer contemplates the situation in his quarters, when Phlox enters, revealing that a wisp tried to possess him but failed. In the armory, another one inhabits Malcolm, who then goes into a turbolift and begins talking to a woman about the differences between male and female anatomy and offering to have sex. He then goes into T'Pol's quarters and does the same with her (also offering to put her consciousness into Malcolm's body), so she calls for Archer and a security team. They demand that the creature leave Malcolm's body, but it refuses, so they confine the possessed Malcolm to his quarters. As Trip goes back to work in engineering, more and more crew members become possessed, so Archer orders security to confine anyone acting oddly to quarters and asks T'Pol and Phlox to find a way of identifying who's possessed.
The engineers manage to restore impulse engines, but Archer decides not to go anywhere until everybody's back to normal. Phlox and T'Pol discover that when someone is possessed, they have elevated levels of acetylcholine in their autonomic nervous system. Travis sees a wisp, runs away, and then finds out that wisps can't enter the catwalk. Then, Hoshi becomes possessed.
All of the non-affected people hide out in the catwalk, except for Phlox because he's immune anyway, and all those who are possessed are confined to quarters. T'Pol wants to go out to find out more about the aliens, then the possessed Hoshi claims she broke her leg. Phlox enters her quarters to find out she was faking and she attacks him, but luckily he manages to sedate her.
T'Pol follows through with her plan to find information and finds out that the aliens lied about wanting to find out about their ancestors— their actual reason for their actions is that they need the ship to survive but it's breaking down and they can't fix it, so they're possessing corporeal lifeforms as a Plan B. Archer, T'Pol, and Phlox then think up a plan to save everybody: flood the ship with carbon dioxide, making everyone suffocate and forcing the wisps to leave, then have Phlox revive everybody.
They set to work on their plan, but a wisp somehow breaks into engineering and possesses Trip. The possessed Trip, who heard about the plan, punches Travis, leaves, and finds Phlox. After a short fight, Phlox opens the CO2 valve, causing the gas to spread, the crew to suffocate, and the wisps to leave. When the crew is revived, the wisps try to recapture Enteprise, but Malcolm has gotten the weapons back online and fires at the ship, destroying it.
Tropes in this episode include:
- Abandon Ship: The wisp aliens want to escape their deteriorating starship, using the Enterprise crew's bodies as their lifeboats - and the swapped crewmates trapped on the doomed ship.
- Ascend to a Higher Plane of Existence: The wisp inhabiting Tucker claims his people were once flesh and blood beings, but they 'evolved' into their present energy form a long time ago.
- Big Eater: Possessed Trip can't resist sampling the entire menu.
- Call-Back: They kept the backup bridge setup in the nacelle catwalk. Trip even mentions the osmium alloy again.
- Continuity Nod: When (possessed) Malcolm visits T'Pol and tells her how beautiful she is, she asks him if he's been drinking. Perhaps she heard about how the last time he got drunk, he started talking about her bum.
- Do You Want to Copulate?: The alien inhabiting Lt. Reed propositions T'Pol and a female ensign.
- Idiot Ball: It takes several seconds and a punch to the face for Travis to realize that Trip just might be possessed.
- Instant Sedation: Phlox disables Possessed Hoshi with his off-button hypospray. Even more impressive as he got her in the lower leg instead of her neck.
- In Vino Veritas: T'Pol assumes that Malcolm's odd behavior is this, before realizing that he's possessed.
- Mega-Maw Maneuver: How the alien ship grabs Enterprise.
- "Not So Different" Remark: Possessed Tucker claims his people are peaceful explorers just like the Enterprise crew.
- O.O.C. Is Serious Business: Quite often, other characters can tell when somebody is possessed by odd changes in behaviour:
- Michael Rostov suspects something is wrong with Trip when Trip, who is usually a dedicated engineer, leaves engineering in the middle of a job.
- T'Pol can tell Malcolm is out of sorts (initially suspecting him to be drunk and then correctly guessing that he's possessed) when he offers to sleep with her and begins talking about the physical differences between their sexes. While Malcolm does find T'Pol attractive, he's usually too straight-laced to come onto a coworker, and he obviously doesn't usually rant about reproductive anatomy.
- Properly Paranoid:
- Archer is suspicious of the aliens from the start, considering they disabled Enterprise and locked it up inside their ship. Of course, he's right and their intentions are definitely not peaceful.
- Phlox as well. When Possessed Hoshi calls and says that she's been injured, he shows up with a phaser and his trusty off-button hypospray. Good thing, as she's not really injured.
- Wounded Gazelle Gambit: Possessed Hoshi pretends to be injured, then when Phlox goes to treat her, she starts fighting him.
- You Called Me "X"; It Must Be Serious: As usual, T'Pol shows how agitated she is when she uses First-Name Basis instead of rank/title."We don't have time for this, Phlox!"
