[s1e14] The War Without, The War Within -

With few options left, Starfleet turns to a desperate and controversial strategy. Rather than face the Klingons in traditional combat, they decide to strike at the Klingon homeworld, . To lead this mission, Admiral Cornwell makes the shocking decision to appoint the Mirror Universe's Emperor Philippa Georgiou as the captain of Discovery, passing her off to the crew as the "rescued" Captain Georgiou. Internal Conflicts and Reconciliation

In the penultimate episode of Star Trek: Discovery ’s first season, the crew returns from the Mirror Universe to a devastating reality: they have overshot their timeline by nine months, and the Klingon Empire has nearly won the war against the Federation.

More information on the behind the mycelial moon-greening.