The plastics firm where Scotty and McCoy bartered the formula for transparent aluminium in exchange for the panels needed to build a whale tank in a Klingon ship.
Plexicorp is a manufacturing company in the San Francisco area visited in Star Trek IV: The Voyage Home. Needing a tank strong enough to carry two humpback whales and the water around them, Scotty and McCoy call at the plant posing as a visiting professor from Edinburgh and his assistant.
The plant manager, Doctor Nichols, shows them his acrylic sheeting. Having no money, Scotty offers something better: the molecular structure of transparent aluminium, a material far stronger and thinner. After famously trying to address a desktop computer by voice, he types the formula in.
McCoy worries about altering history, and Scotty suggests that Nichols may have been the inventor all along. The company supplies the panels, which Sulu ferries to the ship by helicopter.