Forget Me Not

As a human host to a Trill symbiont, Adira cannot remember becoming the host of the Tal symbiont, and cannot access the memories of previous hosts such as Senna. Burnham takes them to the Trill homeworld in hopes that the Trill can help unlock their memories. The Trill are also no longer part of the Federation, but welcome Discovery due to their need for new symbiont hosts. However, many Trill refuse to believe that a non-Trill could host a symbiont, and Adira is turned away. A group of Trill attempt to kill Adira and take the symbiont, but Burnham stops them and a friendly Trill, Guardian Xi, offers to help. At the sacred Caves of Mak'ala, Xi and Burnham help Adira connect to Tal's memories: when Senna died, Tal was transferred to Gray, Adira's Trill boyfriend.

Summary from Wikipedia, CC BY-SA 4.0.