Brothers Johnny and Scotty find Sigmund, a sea monster, on the beach and bring him home to their clubhouse. They become fast friends, but when a monster hunter, Capt. Barnabas, finds out, the kids have to scramble to keep Sigmund hidden. Worse still, Sigmund's brothers Blurp and Slurp don't like humans. Against all odds, can they still have an endless summer of fun?