Montgomery Scott was the chief engineer of the ISS Enterprise.
In 2267, Scott, James T. Kirk, Nyota Uhura, and Leonard McCoy were transporting aboard the ISS Enterprise following negotiations on the Halkan homeworld. While attempting to return their vessel, an ion storm interfered with the transport and Scott and crew were exchanged with their counterparts from the mirror universe, arriving aboard the USS Enterprise. Scott and the three others were immediately placed in the brig of the USS Enterprise. Soon after they were returned by the Spock of that universe. (TOS: "Mirror, Mirror")