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From Memory Alpha, the free Star Trek reference

"Wisp. Interesting word. Wisp. Wisp. Yes, I suppose you could call us that. Wisp."

Commander Tucker, possessed by a "Wisp."

The energy life forms, called "Wisps" by the crew of Enterprise, originated as corporeal lifeforms of "substance" that evolved into a form of non-corporeal, massless "perceptive energy" that existed and traveled through subspace. In their non-corporeal form, they contain no elements that could be identified by mid-22nd century Earth Starfleet sensors.

In 2151, Enterprise was encountered by the Wisp's ship, which opened up a large bay door, and "swallowed" the Enterprise.

Inside, the "Wisps" flew freely in the ship's atmosphere, which consisted of helium with traces of xenon. Upon their first encounter with Humans, the "Wisps" converted their atmosphere to one of nitrogen and oxygen. Shortly thereafter, they briefly inhabited Charles Tucker, before slowly taking over crew members, one by one, aboard Enterprise. Based on Tucker's experience, the "Wisps" could swap places with a body's owner, who then spends time as a similar, but yellow in color wisp-creature. While they are in this state the humans were immersed in realistic hallucinations which are probably self-driven fantasies.

After having released Enterprise from their ship, the "Wisps" traveling briefly through space and began entering Enterprise through the hull. First contact was then made through Tucker, who explained to Jonathan Archer that they were explorers, who wished to explore the corporeal existence of their ancestors.

Once a number of crew members were taken over by the "Wisps", Phlox and T'Pol found a way to determine that those in the possessed state were characterized by elevated acetylcholine levels in the autonomic nervous system.

Eventually, and by accident, it was discovered that the "Wisps" were unable to penetrate the osmium alloy of the hatchway into Enterprise's catwalk. As a result, the crew took refuge in the Catwalk, while they attempted to formulate a means to defeat their invaders.

When the "Wisps" attempted to enter T'Pol, she was able to repel them with her mental discipline, at which point she discovered that they were lying, and were intending to take over the ship.

Finally, it was discovered that the "Wisps" could be expelled by suffocating the "hosts" with carbon dioxide, Phlox flooded the ship with CO2 and freed all the crew of the "Wisps" influence. Enterprise quickly pulled away from their ship and destroyed it by firing two spatial torpedoes into the "belly of the beast." (ENT: "The Crossing")