A ship feeding off Enterprise 's warp field is exposed after the crew ignites the plasma exhaust. Commander Tucker takes a trip to the ship to help them repair their warp drive. After his return to Enterprise, he learns that he is pregnant.
In Engineering, Commander Tucker is trying to solve ship-wide problems with power supply and life support. Sub-Commander T'Pol discovers a distortion in the wake of Enterprise. Captain Archer ignites the plasma, revealing a cloaked ship. He hails the vessel and a Xyrillian, Captain Trena'l explains that they have been tapping Enterprise 's exhaust to recharge their engines. Archer offers assistance. Tucker is to visit for three days, and he is given injections by Doctor Phlox so his body can adapt to the alien environment.
Once aboard, Tucker meets Ah'len, the Xyrillian engineer. They become evermore friendly while repairing the ship. Ah'len takes him to a holodeck to show him Thera, the Xyrillian homeworld. During the tour, Ah'len engages in a seemingly innocent game that allows them to share their thoughts and desires. When the repairs are completed and Tucker returns, he notices a strange growth on the inside of his wrist. He visits Phlox, who tells him that he is pregnant. Tucker mentions the telepathic game and Phlox says that it might be the transfer medium for the alien fetus. Later, as the embryo grows, Archer asks T'Pol to locate the Xyrillian ship.
They discover it hiding in a Klingon warship's plasma wake. Archer hails Captain Vorok, and helps them to detect the Xyrillian ship. Angry, and wanting to destroy the ship, T'Pol reminds him that Archer was the one who found Klaang and saved Qo'noS from civil war, and that the Chancellor himself called him "brother" in the Klingon High Council. Tucker goes to the Xyrillian ship, and Ah'len notes the embryo is young enough to be safely transplanted. She apologises to him, informing him she was not aware pregnancy was even possible with another species. Vorak ends the encounter after securing the Xyrillian's holodeck technology, but warns Archer against a future meeting. T'Pol later reminds Tucker that his was "the first recorded instance of a human male pregnancy".
Staff member André Bormanis later explained that the inclusion of a Klingon battlecruiser design that had been used previously in the franchise (in time periods set later in the timeline) was to indicate the slow evolution of Klingon ship design. He compared this to the speed of changes seen in the rockets used by the Russian Space Agency, and said that the philosophy for the Klingons was that "if the design works, why change it?"