A ship feeding off the 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 the Enterprise, he finds some unexpected developments both on and inside his body.
In Engineering, Chief Engineer Trip Tucker is trying to solve ship-wide problems with power supply and life support. T'Pol discovers a distortion in the plasma wake of the Enterprise. Archer ignites the plasma waste, revealing a cloaked ship. Captain Archer hails the vessel and the Xyrillian Captain Trena'l explains that they have been tapping the Enterprise's plasma wake to recharge their engines. Archer offers engineering assistance. Trip is to stay on the Xyrillian ship for three days until the repairs are completed. Because of the different atmosphere in the Xyrillian ship, Trip is given injections by Dr. Phlox so his body can adapt.
Once aboard, Trip meets Ah'len, the Xyrillian engineer. Trip and Ah'len become friendly while repairing the ship. While waiting for the teraphasic coils to recharge, Ah'len takes Trip to a holodeck to show him Thera, the Xyrillian homeworld. During the holographic tour, Ah'len engages in a psychic game with Trip that allows them to share thoughts. When the repairs are completed, Trip returns to the Enterprise. Trip notices a strange growth on the inside of his wrist, and visits Phlox, who tells him that he is pregnant. Phlox explains that the Xyrillians only utilise genetic material in reproduction, the males acting as hosts. He also reveals that the embryo cannot be removed without risking both of their lives. Trip mentions the telepathic game and Phlox says that it might be the transfer medium for the genetic material.
Archer asks T'Pol to locate the Xyrillian ship, which Reed detects, only to discover it hiding in a Klingon warship's plasma wake. Archer hails the Klingon ship, which fires two warning shots. Archer hails again and asks Captain Vorak if they have been experiencing any technical problems. The Klingons detect the Xyrillian ship with Archer's help, but then order that the Xyrillians must be executed. T'Pol interrupts to tell Vorak that Archer is the man who found Klaang and saved Qo'noS from civil war and that the Chancellor himself called Archer brother in the High Council Chamber. Archer convinces Vorak to take Trip to the Xyrillian ship, but only after informing him of Trip's pregnancy. There, Ah'len tells a relieved Trip that the embryo can be safely removed.
Back on their ships, Vorak ends the encounter by stating that the debt to Archer is repaid and that Archer will regret meeting the Klingons if they ever cross paths again. In the Captain's Mess, Archer asks T'Pol if the Klingon Chancellor called him "brother." T'Pol replies by saying that Klingons are known to exaggerate, so she saw nothing wrong with doing the same. T'Pol tells Trip that his "pregnancy is 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?"