Beverly Crusher becomes romantically involved with Trill ambassador Odan, who is being escorted by the U.S.S. Enterprise to mediate a dispute in the Peliar system. When he is fatally wounded, she finds the man is the host part of a joined species consisting of a humanoid host and a symbiotic parasitic being. To complete the mission, Riker volunteers to be the temoporary host until a new host arrives from Trill.

Franc Luz (Ambassador Odan)
Barbara Tarbuck (Governor Leka Trion)
Nicole Orth-Pallavicini (Kareel)
William Newman (Kalin Trose)
Patti Yasutake (Nurse Alyssa Ogawa)
Robert Harper (Lathal Bine)

• Odan is mediating a dispute between the Alpha and Beta moons of Pelia Zel.
  
• This episode introduces the Trill "joined species," the species of Lt. Dax of Deep Space Nine. However, they seem to be very different species. Odan has ridges on his forehead, while Dax and every other Trill has spots. Odan says a transporter will damage the Symbiont, while Dax transports all the time. Odan implies that all of the Trill are "joined," but in the DS9 Trill, only 10% of the Trill are joined. Odan's symbiont also seems fond of pushing out on his abdomen. We have never seen this in any of the DS9 Trills. The symbiont here completely takes over the host's mind. In DS9, the symbiont combines with the host to create a new individual. In this episode, Troi and Picard claim they know little of the Trill. However, Dax is a Starfleet officer, and her former host Curzon Dax served with Sisko and they were very good friends. Trill is a Federation world. The DS9 producers said they made the changes to make the Trill more practical.
  
• The replacement host sent by the Trill is a female. Beverly admits that she loves Odan, but can not cope with this kind of change, despite Odan's wishes to continue seeing her. (In the modified Trill species, a relationship with a previous host's lover is forbidden; the punishment is exile, with the eventual death of the symbiont when the host dies.)
  
• Troi and Crusher are seen taking advantage of the ship's salon, which has a new Bolian barber.