The show centered on two brothers named Johnny (Johnny Whitaker) and Scott Stuart (Scott Kolden). While playing on the beach, the two of them discover a friendly young sea monster named Sigmund (performed by Billy Barty) who had been thrown out by his comically dysfunctional undersea family for refusing to frighten people. The boys hide Sigmund in their clubhouse.
Plotlines were very simple and straightforward, usually some variation on the idea of Sigmund doing something silly to arouse attention, and the boys working to prevent him from being found by Sigmund's brother monsters Blurp (performed by Bill Germaine and Larry Larsen, voiced by Walker Edmiston) and Slurp (performed by Fred Spencer and Paul Gale, voiced by Walker Edmiston) who want Sigmund to scare people in order to impress their parents Sweet Mama Ooze (performed by Van Snowden, voiced by Sidney Miller) and Big Daddy Ooze (performed by Sharon Baird, voiced by Walker Edmiston). The brothers also worked to hide Sigmund from their overbearing housekeeper Zelda (Mary Wickes), elderly neighbor Mrs. Eldels (Margaret Hamilton) and Sheriff Chuck Bevans (Joe Higgins). Zelda was the boys' housekeeper while their parents were away on vacation. Strangely, the parents were never seen on the show, nor did they return home by the end of the series.