One Mysterious Night (1944)

"What Happened to the Fabulous Diamond "Blue Star of the Nile"?..."

After a rare gem is stolen from an exhibition at a posh hotel, Inspector Farraday decides to recruit former thief Boston Blackie to find the stone. Along with his assistant, "The Runt", Blackie focuses his investigation on the hotel manager, George Daley, and his sister, Eileen. Through disguises and ruses, Blackie and the Runt try to trick their way to discovering the thieves.