A Case of Chiaroscuro

Number One, M'Benga, and Uhura enter the Klingon-occupied Mat'aar system on a shuttlecraft to recover a missing Starfleet probe. They succeed, but are detected and need to hide on the main planet, where color perception is blocked and the residents dress like those in 1940s Earth. Due to a fortuitous resemblance, Number One assumes the role of Nevette, a Mat'aaran who is both a prominent collaborator and the leader of a sophisticated Mat'aaran resistance organization. Uhura is arrested and interrogated by Kor, the local commander, and is later released but learns the current Klingon plan. Over M'Benga's advice, Number One carries out Nevette's previous plan to disrupt that plan and allow some Mat'aarans to escape the planet. On the Enterprise, La'An suspects that the recovered probe has been sabotaged.

Summary from Wikipedia, CC BY-SA 4.0.