Big-hearted Prue, living in the slums, and Danny O'Maddigan, a reformed crook, want to buy a birthday cake for Prue's 75-year-old grandmother. They live across the hall from Ellen Rutherford, the destitute widow of Steven Rutherford, Jr., who was disinherited by his father, a wealthy candy manufacturer. Prue, who works at the candy factory, gives Ellen the money that was meant for her grandmother's cake so Ellen will be able to care for her son Bobby. Frustrated over the loss of the money, Danny steals the price of a cake from the factory's safe, leaving the safe's door open in his haste to depart.

Cyrus Baird is trying to force his daughter to marry Ethelbert Granger, wealthy but effeminate. She meets and falls in love with James Morton, nephew of Baird's pet enemy. Ethelbert takes the Bairds to cruise in the houseboat of Max Rupholdt, who has a mine layer concealed in the innocent looking craft. Morton, suspected of disloyalty, penetrates Max's secret and rescues the girl just before the fleeing spy meets extermination through one of his own mines.