Ep 4. Equilibrium

Dr. Julian Bashir is surprised when he is contacted by one of his former medical professors, Dr. Lenara Kahn. Dr. Kahn is a Trill, a humanoid species that lives symbiotically with a species of small, worm-like creatures, known as symbionts. The symbionts live for hundreds of years, and when a Trill dies, if the symbiont is still viable, it is given to a new host. This means that the memories, knowledge, and experiences of each host are retained by the symbiont, allowing each Trill to build on the achievements and experiences of previous hosts.

Dr. Bashir and Dr. Kahn had a romantic relationship during their time together at medical school, which ended when Dr. Kahn left to complete the rigorous training required to become a host. It is revealed that Dr. Kahn's current host, Odan, was once the host of a close friend of Dr. Bashir's, who died some years ago.

Dr. Kahn is on the station to study a rare subspace phenomenon, and she and Dr. Bashir rekindle their relationship. However, things take a darker turn when Dr. Kahn's symbiont begins to experience problems, causing Dr. Kahn to suffer from erratic mood swings and memory loss.

Dr. Kahn fears that her symbiont may be dying, which would mean that she would also die, as the symbiont cannot survive independently. In desperation, she turns to Jadzia Dax, the station's Trill science officer, for help.

Jadzia discovers that Dr. Kahn's symbiont is not dying, but has been purposely suppressed, with the memories and personality of its previous host erased. This is a violation of Trill law and tradition, as hosts are supposed to build on the experiences of previous hosts, rather than erase them.

Jadzia and Dr. Bashir investigate further and discover that Dr. Kahn's symbiont had been intentionally suppressed by one of Dr. Kahn's own family members, who was trying to protect a dark secret from their shared past.

As the truth emerges, Dr. Kahn is forced to confront the fact that her previous life was not what she thought it was, and that her memories of a happy childhood were false. The suppressed memories, however painful, are eventually restored to the symbiont, restoring Dr. Kahn's mental stability.

In the end, Dr. Kahn decides to relinquish her position as a host, feeling that she can no longer be part of a society that forces its members to suppress the truth in the name of tradition. Jadzia Dax, meanwhile, is left to ponder the deeper meaning of her own existence as a Trill, and what it means to carry the memories of all her previous hosts.