Johnny and Scotty befriend Sigmund, a sea monster, on the beach and bring him home to their clubhouse; Captain Barnabas is hunting Sigmund; Sigmund's brothers dislike humans.

Sigmund and the kids set out to find one another; Sigmund's overprotective family and Captain Barnabus make things hard for Sigmund and the kids.

After the kids teach Sigmund about calling dibs, he mistakenly thinks he can take anything he wants.

After watching a monster movie, Johnny and Scotty decide to make a movie of their own with Sigmund; Robyn helps Maxine run a support group to cure Barnabus of his sea monster obsession.

Sigmund volunteers to help Johnny and Robyn with the annual sand castle contest; Sigmund gets sick; Scotty brings a robotic vacuum to clean the cave.