The Enterprise encounters a new world much like Earth. A small crew stays on the surface overnight to continue researching the planet. During the night, a violently windy storm brews and forces the crew into a local cave. There, in the cave, crew members believe they are not alone.
While headed for an unnamed nebula, Enterprise unexpectedly passes an unknown M-class planet with an Earth-like atmosphere and surface. The Enterprise enters orbit and Captain Jonathan Archer orders a shuttle to be prepared for an away mission. After an afternoon on the planet, Science Officer T'Pol requests that Crewmen Cutler and Novakovich remain with her to study several nocturnal marsupial species the landing party has discovered. Trip and Ensign Mayweather also request to remain on the planet, with Archer's approval. The captain and Reed then return to Enterprise.
Later that evening, a storm front approaches so Trip suggests to T'Pol that the landing party use the cave that she discovered earlier in the day for shelter. Mayweather goes back to the original camping site to recover food, and notices some humanoid lifeforms wandering around. In the cave, Novakovich notices movement in the back of the cave too, then becomes paranoid and runs out of the cave. Trip and Mayweather follow with phase pistols. T'Pol takes another pistol and walks in to the interior of the cave to look for other lifeforms, leaving Cutler alone with the remaining pistol. Cutler thinks that she sees T'Pol in conversation with an alien humanoid. However, when she moves closer T'Pol is on her own.
Trip reports in to Archer about his seeing an alien life form and losing Novakovich, they return to the cave where Cutler voices her concerns. The landing party are now suspicious of T'Pol, thinking that she is keeping information about the imaginary lifeforms from them. Archer and Reed attempt to reach the landing party in a shuttlepod but cannot until the wind dies down. Novakavich is beamed up to the ship due to his erratic bio-sign, where Dr. Phlox finds he has been subjected to tropolisine, a hallucinogenic compound found in the pollen on the planet. Meanwhile, Trip is suffering from paranoid delusions, shooting at the imaginary aliens in the walls. T'Pol uses the distraction to retrieve a phase pistol and levels it at Trip.
Phlox discovers Novakovich is dying, poisoned by an unexpected side-effect of tropolisine. T'Pol reports to Archer that Trip is irrational and that Mayweather and Cutler are nearly unconscious. Reed beams the antidote down to the cave, and Archer lies to Trip about a secret mission from Starfleet, to persuade him to lower his weapon. Trip does and a recovered T'Pol stuns him. She then retrieves the antidote and injects the landing party. The next morning, the storm is over, and everyone is fine.