An enormous alien ship that swallows Enterprise whole, carrying non-corporeal beings who try to take over the crew's bodies because their own vessel is failing.
The wisp ship appears in the second-season Enterprise episode The Crossing. It overtakes Enterprise with ease and draws it inside a cavernous interior where the engines and weapons stop working. Glowing wisps then begin to pass through the hull and exchange places with crew members, who report blissful out-of-body experiences.
The visitors claim to be explorers who are curious about physical life. T'Pol, letting one into her mind, discovers the truth: their ship is deteriorating and they intend to occupy all eighty-odd bodies permanently. The catwalk in the nacelles proves to be shielded against them.
Phlox, whose Denobulan physiology resists the takeover, floods the ship with carbon dioxide to drive the wisps out of their hosts. Once free, Enterprise turns and destroys the huge vessel with torpedoes.