Mermaid Angel Tritone escapes a shark by jumping into single dad Logan Hardington's fishing boat. All his young son Michael wants for his birthday is a mermaid, and if his dad will only play his cards right, now he'll have one.

Angel wants to give Michael his most beloved birthday gift (a real live mermaid) while also getting to know his father and getting him to help her in her mission to stop humans from destroying the oceans. Logan has never met a woman who cared as much about the same things he does, and she's never met a man who understood her deepest commitment to humanity and nature. When an enemy of conservation shows up and tries to kill Angel, Logan has to choose between continuing the life he's known, or following the woman he loves to the bottom of the ocean.