Nelly and her family are camping next to an unusually deep lake. It is said that a monster lives in the lake, but it’s probably just a great big pike, laughs Daddy as he plans to catch it. Nelly doesn’t know what to believe, but one thing she does know for sure: the lock keepers Bror and Elsa are behaving very oddly. As they guide boats through the lock between the lake and the sea, they keep banging the bottom of the lock with long poles - as if they were searching for something. And why are they stuffing sedatives into freshly cooked chickens? There’s a lot to investigate. It’s lucky that Nelly and her dog London have some help from her new friend Walle and his basset hound Ebba.