The attempt by non-corporeal beings to take permanent possession of the Enterprise NX-01 crew's bodies after their vast ship swallowed the Earth vessel.
The incident occurs in the Enterprise episode The Crossing. A huge alien ship overtakes Enterprise and draws it inside. Wisps of light then begin entering crew members, swapping minds with them so that the visitors can sample physical life while the humans drift in a bodiless state.
Archer becomes suspicious, and T'Pol, briefly possessed, learns the truth: the aliens' ship is failing, and they mean to abandon it and keep the crew's bodies for good. The catwalk in the nacelles proves to be shielded against them, and the unaffected crew take refuge there.
Phlox, who is immune, floods the ship with carbon dioxide, driving the beings out of their hosts. Enterprise breaks free and Archer destroys the alien ship with torpedoes, a choice that ends the threat and several hundred lives.