Beverly Crusher becomes romantically involved with a Trill ambassador who is being escorted by the U.S.S. Enterprise to mediate a dispute in the Peliar system. As the starship approaches its destination, Riker volunteers to shuttle the ambassador, Odan, to Peliar to meet with representatives of the planet's Alpha and Beta moons, which have armed for war against each other. Shortly after Riker and Odan depart, a ship opens fire on them, critically injuring Odan and forcing their return to the U.S.S. Enterprise. Back on board, Beverly's examination of Odan indicates that a parasite is invading his body. As she prepares to remove it, she is shocked when Odan stops her, revealing that he is the parasite, and his body is merely a host. The parasite, not the body, is what must be saved.

While Beverly struggles to accept the fact that the handsome man she fell in love with is actually a small purplish lump of tissue, the Enterprise contacts the Trill for a new host body for Odan. Unfortunately, the situation in the Peliar system cannot wait and requires Odan's immediate attention. With this in mind, Riker volunteers his body to serve as temporary host to Odan so he can complete his mission. Beverly is successful in transferring Odan into Riker's body, but cannot get used to seeing him as Riker and is unable to relate to him as a lover. Odan sadly agrees to stay away from her if his presence causes her pain.

Soon after the transfer, Riker's body begins to experience severe physical pain. Odan proceeds with the meeting anyway, and learns that Beverly's discomfort with the notion that he exists within Riker is shared by the representatives of Peliar's moons. They suspect the odd scenario may actually be a plot by Starfleet concocted to enable the organization to impose its own agenda on the Peliar situation. Luckily, Odan is able to convince the Beta representative to accept him and proceed with the negotiations, and the Alpha representative agrees to have an answer within eight hours. Later that day, Beverly arrives at Odan's quarters. Overcome by her desire for Odan, she looks past Riker's body and falls into her lover's arms.

The next morning, as he prepares for the mediation, Odan tells Beverly that his presence has become a threat to Riker's body, and makes her promise to remove him after the meeting. The dispute is settled quickly, and Odan returns to Beverly to be surgically removed from Riker's body. The operation goes well, but Odan's own life is endangered when the Trill ship carrying his new host body is delayed. The U.S.S. Enterprise sets off at warp speed to intercept the ship and save Odan. As the situation becomes critical, Worf announces that the host has arrived and Beverly is dismayed when he ushers in a beautiful young woman. After she transfers Odan into the female body, she sadly explains to her lover that she cannot adjust to the constant change and uncertainty and must end their relationship. Odan accepts her decision and, after the two exchange vows of love, Odan returns to Trill.