Dax's memories uncover a secret about the Trill.
Dax starts playing an unfamiliar tune on the piano and is troubled by mood swings and visions of a masked figure. Sisko and Bashir take her to Trill, where the Symbiosis Commission is evasive, and her condition worsens to the point that doctors plan to remove the symbiont, which would kill Jadzia.
They find that the music was written by Joran Belar, a violent man whose records had been erased. Joran had been secretly joined to Dax, and his memories had been suppressed. The Commission had hidden this because the fact that Joran survived as a host proves that far more Trill can be joined than officially stated. Jadzia accepts Joran's memories and recovers.