A large alien vessel overtakes and swallows Enterprise. Enterprise's engines and weapons are disabled, and the ship and its energy-ball-like beings don't scan well, so Archer takes Reed and Trip on an away mission that finds Trip being invaded by one of the wispy energy balls before being returned to his normal state, albeit with memories of having been on Earth. Phlox's examination reveals no ill effects for the experience.
Trip is possessed again, and is tracked down in the mess hall where he's been enjoying the sybaritic pleasure of food. The alien says that they are benign explorers who want to experience the corporeal state they long evolved from. In exchange, the crew would have wonderful experiences. Archer demands the release of his ship, and the return of Trip -- which quickly happens… for the most part. The Enterprise is again floating in space, but the engines are still not working.
One-by-one, crewmembers start being possessed and acting out of character. Travis discovers a sanctuary in the nacelle catwalk area, so Archer moves the non-infected crew back onto the cramped environs of the engines. While there, T'Pol proposes a plan that she will use her more disciplined mind to try to communicate with the beings instead of being infected by them. Archer objects to this dangerous plan, but there are no other alternatives.
T'Pol discovers that the aliens plan to co-opt corporeal bodies to save themselves from their failing ship. The E-Team hatches a plan to eject the beings by asphyxiating the infected crew. Though he has some trouble with an infected Trip, Phlox floods the ship with CO2, and the aliens leave. But not far enough, as Archer torpedoes the vessel, destroying it.