Saints of Imperfection

Stamets and Burnham conclude that Tilly has been taken into the mycelial network. She wakes up there with the May parasite, who wants her to help stop a "monster" ravaging their world. Discovery finds the shuttle Spock used to escape the psychiatric unit only to discover Georgiou onboard. Section 31 Captain Leland assigns Tyler to Discovery as a liaison to ensure that Discovery does not interfere with Section 31's own investigation into Spock. Discovery conducts a half-jump into the mycelial network to give Stamets and Burnham limited time to find Tilly before the network consumes the ship. Burnham and Stamets discover the monster is Stamets' husband, Hugh Culber, the former Discovery Medical Officer who Tyler murdered during the war. Stamets was connected to the network when Culber died, allowing Culber's energy to be recreated by the spores.

Summary from Wikipedia, CC BY-SA 4.0.