Janet MacLachlan played Lieutenant Charlene Masters, the engineering officer in charge of the Enterprise's dilithium crystals, in the first-season episode The Alternative Factor.
Janet MacLachlan (1933–2010) was an American actress with a long list of film and television credits, among them the film Sounder.
In The Alternative Factor she plays Lieutenant Charlene Masters, who supervises the re-energising of the ship's dilithium crystals. She reports directly to Kirk on the state of the engines and is attacked twice when Lazarus breaks into her section to steal crystals for his own vessel.
The role is notable for showing a black woman as a senior engineering specialist in 1967. In the episode Masters wears a blue sciences uniform, although her duties are those of an engineer, and Scott does not appear in the story at all.