A strange godlike being who traveled from the delta quadrant to test the mercy of humanity brings the Tuvix flower with him and spreads it aboard the enterprise. Most of them are vacuumed up by ensigns but one makes it into the teleporter when Scotty and Spock are transporting. Creating Scock. The rest of the episode is about convincing this new live form to call himself Spotty
There are some fantastic comics inolving the tos mirror universe that take place in and were released between wrath of khan and search for spock that are retconned as hell by search for spock but they actually roll with it and work it into the plot. The marvel star trek comics from the 80s aren't too bad. They're kinda bad, but they're light fun that weages around whatever the movies decide happens oddly well