The Enterprise is on an urgent diplomatic mission: to assist in mediating a peace treaty between two historically hostile races. Dr. Crusher is drawn to the charismatic Trill negotiator Odan, who is the mediator chosen by Starfleet.
As the story progresses, the negotiator is struck down when a dissident faction attacks the shuttle he is on, and Dr. Crusher discovers that he is symbiotically linked with another life form residing within his body - a Trill.
She still remains loyal to Odan despite his alien circumstances as a symbiont entity, and attempts to repair the damage to the host, but she is unsuccessful, which breaks Crusher's heart.
Given the importance of the negotiations, and with the host dying, Commander Riker volunteers to be the new host for Odan, while Odan's homeworld sends a new host. Odan in Riker's body confesses Beverly that he still loves her. Beverly is very puzzled: were it Odan's eyes and hands that she loved or was it something more? Finally she decides to continue in this love relationship.
Riker is the first human to ever host a Trill symbiont and his condition deteriorates rapidly. This upsets Crusher so much that Picard sees it and lets her cry in his arms, offering his consolation.
Once the negotiations are completed, Riker is no longer able to continue as a host for the trillâhis life hangs in the balance. The ship carrying the new host experiences engine difficulty as well, so the Enterprise must rush to rendezvous with the Trill vessel.
Crusher anxiously awaits this event, as she fears that Odan too will perish without a stable host. She tearfully confides in Deanna over tea in the Ten Forward lounge and Deanna consoles her with the knowledge that even with loss, the person's love still lingers.
When the new host arrives, to Crusher's horror and confusion, the host is (outwardly) female: apparently, the Trill themselves have no preferences or limitations about their hosts's orientation, for it has no bearing on their compatibility as a host.
Crusher quickly transplants the Odan symbiont into the new host nevertheless, and both seem to be in good health, as well as Riker. Later, the "new" Odan visits her, explaining that Odan still feels love for her and the host understands how strong their feelings once were, but Beverly kindly, but honestly tells her that she is uncomfortable with a seemingly ever-changing host, and cannot commit, although Beverly still loves her. The episode ends on a bittersweet note with Odan nodding in understanding and kissing Beverly's hand before she leaves, telling Crusher, "I will never forget you."