Star Trek: 10 Classified Starfleet Secrets
3. The Mirror Universe Visitors From Discovery
At the end of the first season of Star Trek: Discovery, the ship and its crew were transported to the mirror universe, the edgy, authoritarian reality, first appearing in the Original Series episode Mirror, Mirror.
Eventually, Captain Gabriel Lorca was revealed to be a mirror universe imposter trying to use Discovery to get revenge on the empress of the Terran empire, the mirror counterpart of Michael Burnham's deceased former Captain, Philippa Georgiou. In the end, Lorca was killed and Georgiou was brought back to the prime universe against her will, where she assisted Discovery in ending the Klingon war, then later started working for Section 31.
Despite Discovery's first season taking place before The Original Series, when Kirk's Enterprise encountered the mirror universe in Mirror, Mirror, they had never heard about it before. This is because Discovery's interactions with the mirror universe were classified on a need-to-know basis. Only high-ranking officers like Captain Pike were briefed on the mirror universe, Lorca's origins, and the true identity of the new Georgiou. When interacting with average people, Georgiou played the role of her prime counterpart, stating that she somehow survived her supposed death.