En route to a potential site for the first in a series of Starbases, Enterprise is hailed by an Orion ship. The ship's commander, Harrad-Sar, has a proposal to make: he claims to know of a planet with large deposits of magnesite and says he will give Archer the coordinates in exchange for a share of the profits. He also agrees to open a dialogue between Starfleet and the Orion Syndicate. To celebrate their new partnership, Harrad-Sar presents Archer with a gift: three Orion slave girls.
Soon, the Enterprise crew begins experiencing difficulties: the men seem preoccupied, aggressive, and delusional, while the women suffer from headaches and listlessness. Even Archer isn't immune — he finds himself falling under the spell of the lead Orion slave woman, Navaar, and begins acting in an uncharacteristically irrational manner. Also affected is Enterprise's chief engineer, Kelby, who resents the fact that Trip hasn't returned to Columbia yet. Kelby soon finds himself intimately involved with one of the Orion women, D'Nesh, who convinces him to sabotage the ship's engines and attack Trip.
Phlox determines that the Orion women have exposed the ship to a very potent pheromone — the only crewmembers who don't seem to be affected are Trip and T'Pol. T'Pol deduces that she is immune because of her Vulcan physiology. When Vulcans mate, she notes, a psychic bond is formed — feelings, thoughts and images can be shared. This, then, is why Trip is immune — and explains the strange, shared daydreams both he and T'Pol have been having.
After discovering that the Orion women have been communicating with Harrad-Sar, Archer confines them to the decon chamber. As Trip and T'Pol rush to fix the ship's engines, Harrad-Sar tracks down Enterprise and confronts Archer. The Orion Syndicate, he says, wants Archer's head — and they don't care whether or not it's attached to his body.
As Harrad-Sar launches an attack on Enterprise, he reveals something to Archer: he is merely a "slave" to the situation. Archer realizes the truth: it is the Orion men, not the women, who are the slaves. Harrad-Sar's ship attaches a powerful grappler to Enterprise, and begins towing it. Meanwhile, the three Orion women escape from decon and head up to the bridge. Archer tries to resist their advances, but the Orions are affecting his judgment -- they convince him to arrest T'Pol, who is working furiously to free Enterprise from Harrad-Sar's grappler. Before T'Pol can be taken away, however, Trip emerges on the bridge and takes Archer down with a phase-pistol. With Archer out of the way, T'Pol takes out Harrad-Sar's engines and weapons, freeing Enterprise. Defeated, the Orion women return to their ship.
As the crew recovers from the effects of the pheromones, Trip finally gets T'Pol to admit that she wants him to stay on the ship. After the two share a tender kiss, Trip tells her that he's asked for a transfer back to Enterprise.