While at warp speed, a huge alien vessel overtakes and swallows Enterprise, disabling the ship's engines and weapons. When the scanners register neither the alien ship nor its numerous energy-wisp-like non-corporeal beings, Captain Archer takes Lieutenant Reed and Commander Tucker on an away mission to the interior. While EV Tucker is visibly "entered" by one of the beings before being returned to his normal state. Doctor Phlox's examination, however, reveals no lingering effects.
Later, Tucker is possessed again, and is soon located in the mess hall enjoying the simple pleasures of food. When asked, the alien says that they are explorers who merely want to experience the tangible nature of the corporeal state they long evolved from. In exchange, the crew would be permitted to have out of body experiences. Archer, doubtful of the aliens' intentions, demands the release of his ship, and the return of Tucker's consciousness—both of which quickly happen. Enterprise is now freed, but the engines are not operational. While affecting repairs, the crew start displaying signs of possession one-by-one. Similar events occur throughout the ship, including the takeover of Reed, Ensign Sato, and 22 others.
Ensign Mayweather, himself escaping a pursuing entity, finds sanctuary in the nacelle catwalk area, so Archer orders the uninfected crew there. Sub-Commander T'Pol then proposes a plan to use her more disciplined mind to try to uncover the beings' intentions. Archer is opposed, but assents given that there is no other alternative. T'Pol is soon overcome and discovers that the aliens plan to co-opt the 82 crew-member's bodies to save themselves from their failing ship. Archer, T'Pol and Phlox then hatch a plan to forcibly expel the beings by asphyxiating the infected crew. Though he has some trouble with an infected Tucker, Phlox is able to reach the atmospheric controls and floods the ship with CO
2, and the aliens are forced to return to their ship. When the alien ship attempts to pursue Enterprise, Archer destroys it with two torpedoes.