The Enterprise crew makes first contact with the Vissians, a species with three genders. Trip meets one of the species' third gender, a Cogenitor, a being used for only breeding purposes. After learning how badly Vissians treat all Cogenitors, Trip attempts to help this Cogenitor, even though it directly interferes with Vissian culture.

Stunned by the death of former Starfleet rival and friend, A.G. Robinson, in a mountain climbing accident, Archer grudgingly recalls their competitive relationship with T'Pol as they probe a possible dark matter nebula during a shuttle voyage. He describes how he and A.G. broke the Warp 2.5 barrier on the first high-warp test flight that saved the NX program almost ten years earlier. After they confirm the first dark matter nebula, T'Pol suggests Archer name it the Robinson Nebula.