The War Without, The War Within

The Discovery is boarded by Cornwell and Sarek, who explain that the Klingon houses remain divided and fight between themselves to see which can destroy more Federation assets. The only safe refuge for Starfleet beyond Earth is now Starbase 1, and they warp there with Cornwell assuming command. After emergency surgery performed by L'Rell, Tyler now has his personality back, but can still access Voq's memories; Burnham is unable to forgive his actions as Voq and rejects him. The crew finds Starbase 1 conquered by a Klingon house, and the remainder of Starfleet's command withdraws to protect Earth. Georgiou tells Burnham that she defeated the Klingons in the Mirror Universe with a surprise attack on their homeworld of Qo'noS, and Cornwell agrees to replicate that now.

Summary from Wikipedia, CC BY-SA 4.0.