Undercover cop Nick Dunbar's brother Matt is accused of killing his teacher, Mr. Bradwood at Adlai Stevenson High School. Nick loses his temper with Hechtor, the detective in charge, and gets suspended. Nick's partner Ed pretends to be Nick's dad to enroll him as a student. Matt gives Nick pointers to get people to talk to him. Nick deals with bullies, girls with crushes on him, teachers and staff who range from quirky to bizarre, and a teacher, Robin Torrence, to whom he is attracted who thinks he is a teen. The turning point in his popularity happens in the classroom metaphor scene, in which E.E. Cummings' poem she being brand new, is used in its entirety. The character Renard, who had a large part in the script but was mostly edited out of the film, makes his only appearance in this scene. Along the way, Nick and Ed narrow the probable motives down to jealousy, the girlfriend of Kyle Kerns or self-preservation. Nick's identity is discovered by Jane Melway, one of the con artist's gang, and it all comes to a head at the Pagan May Fest. Nick and Ed find out that they are probably on the wrong track - Chet Butler, one of the gang's members is missing and an emotional confession from Melway points to Butler as the murderer - not to stop the blackmail, but because Bradwood was engaged to Melway, with whom Butler used to have an affair and whom he still loves. Bradwood also found the love letters in which Butler claims to have killed Melway's husband. Butler appears and implicates himself with words and actions. A chase ensues, ending with Nick being cornered finding the evidence that cements Butler's guilt. Nick is saved twice in quick succession, once by a schoolmate's opportune distraction, and once by an excellent shot of Ed's at the perfect time. Nick makes a date with Robin Torrence, who now knows he is an adult, and the real-estate scammers are arrested. Hector looks very foolish. Matt is released from jail and can't wait to get back to school.
Plain Clothes was not widely distributed or reviewed, with critics responding negatively to the film. In her review for The New York Times, Janet Maslin wrote, "Plain Clothes tries to combine a police investigation story with the usual classroom and locker-room stuff. Less would not necessarily have been more, but it would have been shorter." In his review for the Miami Herald, Juan Carlos Coto wrote, "Potentially hilarious moments seem to straggle by like students playing hooky, and the whodunit plot gets dumber and more contrived at every turn."
