The Bowery Boys--Slip, Sach, Bobby, Whitey & Chuck--start their own exterminating service, and get a job which takes them to a spooky old abandoned mansion in the middle of the night. Meeting up with pal Gabe and his new French bride, the boys are tormented by mad scientists who try to convince them the place is haunted and then kidnap Sach in order to place his brain inside a gorilla.

In the slums, teenager Frankie Warren hangs out with a rowdy gang who one day knock him out in a fight. In court, the boys refuse to reveal who struck the knockout blow, and all are subsequently sentenced to a reformatory, cruelly run by two corrupt guards. New deputy commissioner Mark Braden arrives determined to change things, but despite the help of Frankie's sister, Mark's reform plans -- and Frankie's future -- may be sabotaged from within.