A little girl named Trisha follows the Stones home from the park and says that she is one of the family.After a long effort to find her real parents,her uncle arrives.He realizes that the orphaned Trisha loves being in a family and that the Stones are fond of her.He arranges it so Trisha can stay with the Stones.Thus a new family member is added.

Mary learns the joys and pains of having a little sister,as she has to baby-sit Trisha on a night when she'd planned to go on a date.

The Stones all end up at Shelter Bay during ""mid-term vacation"", better known today as ""Spring Break"". Mary doesn't know the rest of the family is there; she thought it was just her and her girlfriends. Donna, Jeff, and Alex get to stay on a fishing boat named ""Big Daddy"", and boy, does Donna hook a big one!

Alex and Donna prepare to go away for the weekend, but Donna suspects Trisha will be sick. Donna sees Trisha eating bananas and she always saw Jeff eating bananas before he became ill. Alex's collegue Dave (Bob Crane) tries to convince them otherwise ... and it turns out that Jeff is the one who came down with a fever. Trisha had been eating bananas because of her promised treehouse and she had wanted to climb trees like a monkey.

There is a local handyman Trisha knows who loves fooling around and making children laugh.Trisha offers him to help fix the little things around the house that need fixing,but it turns out that he is clumsy and inept.Donna and Mary feel helpless as he wrecks much around the house,but soon afterwards the ice-cream man arrives.He knows much more about reapir work- and he hates kids.Donna makes a suggestion,which is followed.The ice-cream man ends up fixing everything around the house-while the handyman ends up delivering the ice cream,and singing to all the kids as they play and have fun.