Sisko summons Dax to his office with some serious news. A group of Trill scientists, led by Dr. Lenara Kahn, will be using the Defiant to conduct experiments. Lenara is a joined Trill who, while her symbiont was joined to a previous host body, was married to one of Dax's former male hosts, Torias. Dax shows some concern, since Trill society expressly forbids relationships with lovers from past lives. The penalty for "reassociation" is exile from the Trill homeworld and the eventual death of the symbiont when the current host dies. However, Dax decides to make the best of it, believing she can control any feelings she may still have for her former wife, and even chooses to attend a dinner reception for the scientists. There, the two women meet at the buffet table and share a laugh. Despite the casual tone of their conversation, however, an attraction begins to develop between the two.

The Trill team — Lenara, her brother Bejal, and Dr. Pren — begins work on the experiment to create an artificial wormhole. As science officer, Dax is involved, which keeps her near Lenara. The women can't resist the urge to talk about the past, so Dax invites her to dinner, with Bashir as chaperone. That night, he politely listens as Dax and Lenara talk about old times and their lives since the marriage of their previous hosts. It's almost a relief for Bashir when he is summoned to the Infirmary, leaving the former spouses alone. Feeling more at ease, Dax takes hold of Lenara's hand, which Pren happens to quietly notice.

When work resumes on the Defiant, the closeness between Dax and Lenara is obvious, reaching a crescendo when the two women hug after the first phase of the experiment proves successful. Soon after, Bejal confronts his sister on the station, and she insists that nothing is going on between the two. Lenara later visits Dax's quarters to tell her what happened. There, the two give in to the building passion between them and kiss. Lenara then forces herself to leave, and Dax remains to struggle with her emotions.

The next day, Dax turns to Sisko for advice. He pushes her to fight the attraction to Lenara out of duty to ensure the continuation of the Dax symbiont. But Dax, realizing that she loves Lenara, is unsure. Later, on the Defiant, the scientists proceed with the second phase of the experiment when their artificial wormhole collapses in a blast of energy which slams into the ship. Lenara is knocked unconscious in the engine room, separated from the others by a roaring plasma fire. Dax risks her life to save Lenara — and the two women realize that they never want to lose each other again.

Afterward, Dax checks on Lenara in her quarters, and the scientist worries about her failed experiment and the extra work she knows she'll have to put into it. Dax seizes the moment and invites Lenara to stay at the station — with her. Lenara, however, cannot bring herself to take that final step, painfully refusing to break their society's rules. Dax desperately asks her to stay, but Lenara leaves, preserving Dax's life — and breaking her heart.