Dr. Crusher falls in love with Odan, a Trillmediator who has been assigned to settle a bitter dispute between the inhabitants of two moons. But after Odan is mortally injured, she discovers that "Odan" is actually a symbiotic worm that lives inside a humanoid host body. He will survive if he's quickly transplanted into the body of another humanoid from Trill, but the Enterprise is a long way from that world. With the dispute between the moons reaching a crisis and Odan weakening, Crusher does the only thing possible. She transfers the symbiosis into a human volunteer: Commander Riker.
The Federation starship Enterprise, under the command of Captain Jean-Luc Picard, brings aboard the charismatic negotiator Odan, a mediator chosen by Starfleet, to negotiate a peace treaty between two hostile races. Chief Medical Officer Dr. Crusher is charmed by the man, and the two share a love affair during the trip. Odan says that he will not transport in the normal way and request that a shuttle and pilot be provided for him; Commander Riker honors this request. At the arranged planet, the shuttle carrying Odan is attacked by one of the dissident factions and Odan is mortally injured. While trying to save Odan in Sickbay, Dr. Crusher comes to learn that Odan is a Trill, a species which symbiotically lives within its host's body. It is a further revealed by Commander Data that the transporter would have harmed the symbiotic lifeform. Though Odan is not harmed, the host body dies soon after this. Commander Riker volunteers to allow Odan use him as a host to conduct the necessary negotiations and sustain the Odan lifeform 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 Troi and wonders about the true depth of her feelings for Odan. Beverly resists Odan's advances because she feels confused and betrayed. From one prospective, as she relates to Troi, she views Riker as a good friend more like a big brother and from the other betrayed because Odan hid his true nature from her. With some effort, Odan in Riker's body, manages to convince the delegates from the warring planets to work with him. However, in time, 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 Picard.
The Enterprise successfully rendezvous in time to bring aboard the new host, a female, 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. However, when Odan, in the female host, attempts to continue their relationship, Dr. Crusher is uncomfortable, knowing both that the Trill appear to have no preferences on gender orientation, and that Odan will continue to live on in any number of hosts' bodies. While Doctor Crusher will only have one body and will not know when the changes will come and fears she won't be able to keep up. Odan admits he 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.
"The Host" is the first appearance of a Trill, although in a somewhat early version.
This episode was a test case for the Gay Star Trek movement, who hoped that Beverly might follow her heart. Gay and gay-friendly Star Trek fans initiated a letter writing campaign to pressure the television franchise to address how the utopian society dealt with sexual orientation and gender identity. The extent to which this episode addressed these issues is a point of contention among Star Trek fans.[original research?]
During the filming of this episode, cast member Gates McFadden was 7 months pregnant in real life, and she is most often filmed from the waist up or wearing her blue doctor's coat to mask her pregnancy.