The fair comes to Beanotown and Dennis is desperate to go, but Mum needs him to take care of little sister Bea whilst she attends her relaxation class. Taking Bea with him, Dennis arrives at the fairground, only to discover that the fair's owners, recalling the chaos he and his friends caused last year, have put 'safety' measures in place which prevent Dennis, Curly, Pieface and Bea from having any kind of fun. Bea threatens to do a poo unless she gets to go on the bouncy castle, so Dennis takes advantage of the operator's tea break and fills the bouncy castle for Bea, unaware that he has used helium by mistake. Caught in the act they flee the scene, only for Dennis to discover that Bea has climbed aboard the now airborne castle. Dennis and Gnasher, Curly and Pieface execute a daring plan to save Bea, but ruins the bouncy castle in the process, landing Dennis in trouble again.