When Robyn gives her mother Maxine a birthday present made by Sigmund, Maxine assumes that Robyn made it and is amazed by Robyn's artistic talent. Not wanting to disappoint her mother, Robyn goes along with the misunderstanding, which snowballs out of control. Meanwhile, Barnabus recruits a reluctant Johnny to help him plan a birthday surprise for Maxine.

Sigmund volunteers to help Johnny and Robyn win the annual sand castle contest. But when Sigmund gets sick in the process, the kids have to figure out how to cure an ill sea monster. Meanwhile, Scotty brings a robotic vacuum to clean their cave, and Slurp thinks it's a pet his mother has finally gotten for him.