The Great Diamond Robbery (1954)

Ambrose C. Park, left on a park bench as an infant with an impulsive need to find his parents, is an assistant to a diamond cutter. Shyster lawyer Remlick, in a strategy to get a fabulous uncut diamond through Ambrose, arranges for Emily Drummon, Duke Fargoh and Maggie Drummon to pose as Ambrose's long-lost parents and sister. The diamond, through many comic situations, is acquired and the gang is going to have Ambrose cut the diamond, and relieve him of the two stones and his parental illusions at the same time. But Maggie, who has no taste for the deception, tips Ambrose off and a wild chase ensues. At the end, Ambrose is very happy as he can now marry his "sister."