Incorporeal aliens encountered in a vast vessel begin taking over the bodies of the crew, intending to use them.
Enterprise is swallowed by an enormous alien ship inhabited by incorporeal beings. The aliens swap places with crew members, taking over their bodies while the humans' consciousness drifts in the alien realm.
Tucker is among the first affected, and the possessed crew behave oddly. T'Pol and Phlox learn that Vulcan mental discipline lets T'Pol resist. Archer discovers the beings intend to occupy the crew permanently, and the crew finds a way to drive them out and break free of their vessel. The episode was directed by David Livingston, with a teleplay by Rick Berman and Brannon Braga.