What’s Past Is Prologue

Lorca frees his old crew, who have been tortured since his disappearance, and with the help of Mirror Stamets they are able to kill those loyal to Georgiou and usurp her throne. She goes into hiding, while Burnham also evades capture and contacts the Discovery. They agree to a plan in which Burnham lowers the containment field around a large energy source in the ISS Charon that originated from the mycelial network. The Discovery will then arrive to destroy the energy source, causing an explosion which they can ride into the mycelial network through which Stamets could navigate them back to their own universe. Georgiou agrees to help Burnham and they attack Lorca's group. Rejected and defeated by Burnham, Lorca is then killed by Georgiou who offers to sacrifice herself to allow Burnham's escape.

Summary from Wikipedia, CC BY-SA 4.0.