It's sometime in the 1920s, when it is illegal, as per the Mann Act, for a man to transport a woman across state lines for immoral purposes, which is what married Nicky Stumpo wants to do with Freddie Bigard. Since he can't marry her himself for that purpose of crossing the state line, Nicky instead enlists his friend Oscar Sullivan, who is trying to evade the local police for embezzlement, to marry Freddie instead. The three, who plan to live together, will masquerade as an extended family under assumed surnames, Freddie and Oscar indeed married, with Nicky Freddie's brother, while behind closed doors Oscar will be the platonic third wheel in Nicky and Freddie's love nest. Although he is purportedly doing it for love, Nicky really wants access to Freddie's money, she being an heiress to a fortune about which Nicky tries to convince Oscar is untrue. The situation becomes more complicated with the combination of Oscar wanting to have his own way with Freddie, as he also does confirm Freddie's own personal wealth, and as Freddie informs her husband and "brother" that she plans on giving away her money. As such, Nicky and Oscar figure the only thing they can do to reach that goal of getting her money is to do away with her permanently - making it look either like suicide or an accident - which is easier said than done in how hapless they are as murderers.
—Huggo