When dorky, unpopular 11-year-old Otis wakes up in the morning at a sleepover birthday party, he's in for a shock. At the marquee where the sleepover took place, it's a scene of carnage and mayhem: there's a donkey drinking at the chocolate fountain, five inexplicable traffic cones are dotted around the sleeping bags, a huge inflatable helium sausage looms above, one of the kids has a new tattoo, and there's a suitcase of bratwurst on the floor. But neither Otis nor the other kids around him can remember what happened!

The party was in the back garden of popular kid Rocco Rococo's huge house, and Otis was only there because his mum forced an invitation. Otis realises that makes him prime suspect for causing the mess, and he needs to clear his name. Meanwhile, Rocco just needs to keep his parents from finding out! So while everyone else makes a befuddled exit, Otis and Rocco reluctantly team up to hide the evidence. They decide to retrace their steps in the hope of solving the mystery of what on earth happened. Did aliens abduct them and steal their memories?

Or maybe they accidentally opened an interdimensional portal and let chaos through? (Nope: as it turns out, it was a party-hire hypnotist with a taste for Late Expressionist paintings and Ming vases: he's been robbing houses and leaving fakes in their place all over town!)