Equilibrium (Star Trek: Deep Space Nine)

Plot

At a dinner party, Jadzia Dax toys with a piano and finds herself playing an unfamiliar tune despite having no musical training in this life nor in her previous ones. Later, she experiences odd mood swings and hallucinations of a masked figure. Bashir finds that these symptoms are linked to the symbiont in her belly. According to Dax's medical history, a previous host experienced similar symptoms when he went into a six-month coma. Sisko and Bashir take Dax back to her homeworld, where Doctor Renhol of the Trill Symbiosis Commission stabilizes Dax's symptoms. But Dax relapses, and experiences a new hallucination of Trill hospital guards accosting her. Frustrated, Sisko, Bashir, and Jadzia visit the caves where unjoined symbionts are spawned to consult the caretakers. A caretaker there believes that Dax's problems stem from memories of one of her symbiont's previous hosts.

Sisko and Bashir identify the musical tune Dax heard in her hallucinations as a composition by a Trill named Joran Belar. When Dax sees his photograph, she experiences another hallucination of the masked man murdering someone, and when she rips off his mask she sees Joran's face. She then goes into neural shock and is hospitalized. Doctor Renhol plans to remove the Dax symbiont, which will kill the host Jadzia. Sisko and Bashir revisit the spawning caves, but the caretaker there has been scared into keeping quiet. Bashir finds that the Trill government deleted information on Joran Belar from its records. However, they manage to contact Joran's brother, Yolad, who says that Joran was a candidate for being joined with a symbiont. According to the official report, Joran was rejected due to his violent temper and murdered the doctor who evaluated him, and was then killed by hospital security. However, Yolad doubts this story, because he remembers Joran telling him that he had been successfully joined. Sisko and Bashir deduce that Joran was one of Dax's previous hosts, and to cover this up Joran's files were altered and Dax's memories of him were suppressed.

Sisko and Bashir confront Doctor Renhol, and accuse her of letting Jadzia die in order to protect an even darker secret. The Trill government claims that only one in a thousand Trill are fit to be joined with a symbiont. If a symbiont is placed in an unsuitable host, both host and symbiont will die in days. Joran was judged suitable and given a symbiont, but by the time his psychological problems were discovered, he had been carrying the symbiont for months. This showed that the criteria for being joined are excessively stringent. Renhol admits that nearly half of all Trill can host symbionts, but if the truth came out the small supply of symbionts would be fought over fiercely, jeopardizing their welfare and destabilizing Trill society. The only way to stabilize Dax's mind would be to allow her suppressed memories of Joran to resurface. Renhol removes Dax's memory blocks after Sisko agrees to keep the truth secret. Dax recovers and comes to terms with her disturbing new memories.