Sisko is making soufflé and other food for the senior staff. Dax arrives late, and start playing a tune on Jake's keyboard, much to her surprise. And that's the pre-credits teaser. Yes.
Later on, while playing chess with Sisko, she starts humming the same tune, much to his annoyance. And then she accuses Sisko of cheating and storms out. Kira goes to talk to her, which doesn't go well either. Dax storms out again, and suddenly finds the Promenade deserted, save for a robed fellow in a mask. Then she runs into Quark, and everything's back to normal. Normal for DS9 anyway.
Bashir gives her a checkup while she apologises to Sisko. Sisko suggests that she put the tune into the computer and see if it can find a match. Bashir says something about one of Dax's previous hosts being injured in a shuttle crash. Apparently her isoboramine levels are low which is something to do with the trill-host interface. Bashir suggests taking her back to the Trill homeworld for the experts to check her out.
Dax isn't that happy about going back to the Symbiosis Commission, because of unhappy memories of the initiation process, and ends up sleeping in Bashir's quarters on the trip.
At the Commission, Dax gets heaps of tests, and they all meet Dr. Renhol, who puts her on some drugs for her isoboramine levels. Shortly after, she has another hallucination of the robed figure and a couple of people struggling with her. Renhol checks her out again, but thinks they're on the right track. Dax decides to try talking to the Guardians, unjoined Trill who care for the symbionts in their pools. Guardians don't get out much.
They talk to Timor, who recognises that something's wrong, and says she's getting memories from a previous host. Meanwhile, the computer's identified the music Dax has been hearing - it's a piece by the Trill Joran Belar. His picture triggers another hallucination, in which Dax sees the masked figure kill a Trill and is revealed to be Belar. Dax collapses, and Renhol says that if things don't improve, they may have to move the Dax symbiont to a new host.
Sisko and Bashir consult Timor, who's now nervous and uncooperative. They also find that Belar's file is almost empty - the information's been deleted. But they find that Belar died the same day as Dax's previous host but one. They try talking to Belar's brother, who says Belar went off to become a symbiont initiate, killed the doctor who recommended he didn't become a host, and was ejected from the program, but that he'd had a phone call from Belar claim to have been joined. Sisko theories that Belar was joined with the Dax symbiont before Curzon.
The pair confront Renhol. Although if a Trill joining was made with an unsuitable host, supposedly both symbiont and host would die within 4 days, yet Dax and Belar were joined for six months, as Bashir points out. Hence the cover up. Renhol says over half the planet's population is capable of joining, but there aren't that many symbionts.
Jadzia talks to hallucinatory Belar, and, er, that's it. They return to DS9 and Dax starts playing the keyboard.