The Fat Man is a 1951 American mystery film directed by William Castle. It is based on a radio drama of the same name, with J. Scott Smart reprising his role as Runyan, a detective.
Portly amateur detective Runyan is hired by dental nurse Adams to investigate the unexplained murder of her boss. Suspicion falls on disappeared patient Clark, who had been helped by probable gangster Gordon. Runyan finds Boyd, Clarks former lover and briefly his wife, who reveals that Clark had done time in jail. The police tell Runyan it was for robbery of half a million unrecovered dollars. In jail, Clark had a cellmate Deets, with whom he shared everything. On release, Deets had claimed Clarks share of the loot from Gordon and had promised to eliminate Clark, whose corpse he left in a burntout truck. When he realised Clark had been having dental treatment, so his teeth could be identified, he killed first the unfortunate dentist and then the nurse as well. Runyan tracks him down and in a final shootout he meets his end.
Source: Wikipedia