Plot summary
Odan (Franc Luz), a mediator, boards the Enterprise to negotiate a peace treaty between two hostile groups. Doctor Beverly Crusher (Gates McFadden) is charmed by the man, and the two share a love affair during the trip. Odan refuses to use the transporter and requests that a shuttle and pilot be provided for him; Commander William Riker (Jonathan Frakes) honors this request. During the mission, the shuttle is attacked by a dissident faction and Odan is mortally injured. While trying to save the alien in sickbay, Dr. Crusher comes to learn that Odan is a Trill, a species which symbiotically lives within its host's body.
It is further revealed by Lt. Commander Data (Brent Spiner) that the transporter would have harmed the symbiotic lifeform. Following the death of Odan's host body, Commander Riker volunteers to allow Odan to use him as a host to conduct the necessary negotiations until a new host arrives.
Odan's presence becomes dominating over Riker, and Dr. Crusher finds herself initially confused when Odan continues to try to engage with her to continue their relationship. Dr. Crusher is puzzled and full of emotion as she later confides to Counselor Deanna Troi (Marina Sirtis) and wonders about the true depth of her feelings for Odan. With some effort, Odan in Riker's body manages to convince the delegates from the warring planets to work with him. However, Riker's body begins to deteriorate due to the incompatibility of different biologies. The ship transporting the new host has encountered engine malfunctions. Dr. Crusher does everything she can to extend Riker's and Odan's chances while the Enterprise races to meet the Trill ship, and has a deeply emotional moment with Captain Jean-Luc Picard (Patrick Stewart).
The Enterprise successfully rendezvous in time to bring aboard the new host, a female (Nicole Orth-Pallavicini), much to Dr. Crusher's surprise. She helps with the other Trill to transplant Odan into the new host, and both Riker and Odan fully recover. When Odan attempts to continue their relationship, Dr. Crusher is uncomfortable because in her eyes Odan is now a woman, and she knows that Odan will continue to live on in any number of hosts' bodies. Odan states she still loves Dr. Crusher, but understands her confusion and discomfort, and promises to never forget her or their short time together. And Dr. Crusher replies that she loves Odan too; in Odan's new, female host body, Odan then kisses the inner wrist of her hand.
Credits
Adapted from Wikipedia contributors, The Host (Star Trek: The Next Generation). Paragraphs and formatting adjusted; illustrations, reference markers, block quotations and non-story material omitted.