In 2256 a spore jump carried the USS Discovery into the mirror universe, where its crew learned Captain Lorca was a Terran and foiled his bid for the throne.
The jump after the Battle of Pahvo did not go as planned. Discovery arrived in a parallel universe ruled by the brutal Terran Empire, with Stamets left in a coma by the strain. To blend in, Michael Burnham posed as the captain of the ISS Shenzhou while the crew searched for a way home.
Burnham found records of the USS Defiant and learned of the mycelial reactor that powered the imperial flagship, the ISS Charon. In the process it became clear that Gabriel Lorca was not who he seemed. He was a Terran from this universe who had crossed over and arranged Discovery's jump so that he could return and overthrow Emperor Philippa Georgiou.
Lorca seized the Charon, but Burnham and Georgiou fought him, and Georgiou killed him. Discovery destroyed the Charon's reactor, which was threatening all mycelial life, and Burnham brought the Emperor back with her. The ship returned to its own universe nine months later than it had left, and found the Federation close to losing the war.