The Crossing

Travelling at warp, Enterprise is engulfed by a gigantic alien vessel and its systems are disabled. Archer leads a team to investigate, and Tucker is briefly taken over by an incorporeal being. Later, possessed again, he is found enjoying the mess hall. The aliens claim to be explorers who want to experience physical existence, and offer the crew the chance to visit their realm in exchange. Archer demands Tucker's release and the ship's freedom, and the aliens agree. But during repairs, crew members are taken over one by one, including Reed and Hoshi. The unaffected crew retreat to the catwalk, which offers protection. T'Pol volunteers to let a being into her mind, and learns their real plan: to leave their failing ship and permanently occupy the bodies of compatible crew. Archer orders the ship flooded with carbon dioxide to drive the beings out. Phlox fights past a possessed Tucker to the controls, the beings flee, and Archer destroys their ship.