Black Widow

A federal investigator tracks down a gold-digging woman who moves from husband to husband to kill them and collect the inheritance.
Federal agent Alexandra Barnes believes that Catherine Petersen is a serial killer who marries rich men and then murders them for their money. But since Catherine is seemingly a master of disguise and has multiple identities, Alexandra can't prove anything with conventional detective work. With no other option, she goes undercover, pursuing the same man as Catherine, and hoping that Catherine will slip up and reveal her true identity.