La historia comienza en el corazón de la vieja Irlanda, donde Jim O’Connell, un patriota irlandés, se queda solo para cuidar de su hija Margaret, a quien cariñosamente llama “Peg o’ My Heart”, tras la muerte de su amada esposa. Pasan los años. Margaret, ahora una joven ayudando a su padre en la granja, es enviada a Inglaterra para vivir con sus ricos parientes, sin saber que se trata de un plan trazado por su padre y un abogado con el fin de cobrar una herencia.
At a party thrown by millionaire Max Goldberg, Charles Howard, an aristocratic young artist, succumbs to the charms of chorus girl Lita O'Farrell, Goldberg's erstwhile girl friend. After a whirlwind courtship, the two marry and then their incompatibilities begin to surface.
Annette Kellerman, the Australian swimming star of the early 1900s, made a number of films, most of them in the 1910s, which displayed her athletic skills. Most of these films were underwater fantasies, and this one was no exception. Here, Kellerman is Merilla, a mermaid who is the "Queen of the Sea." Not satisfied with being a mermaid, she wants a mortal human body with an immortal soul. She discovers she can achieve this if she saves four human lives.
Little Kate and Janie O'Dowd are sent to their wealthy American uncle, Michael O'Dowd, after their Irish father loses his life on a World War I battlefield. Having been locked accidentally into O'Dowd's munitions plant one evening, the children catch sight of their intoxicated cousin Miles O'Dowd admitting two men into the factory. The girls recognize the two as spies they had seen on the boat to America sending signals to a German submarine. After the spies knock Miles cold, the children trap them in a die-stamping machine until help arrives.
