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 recruits former thief Boston Blackie to retrieve 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 trick their way to discovering the perpetrators.