Dennis is desperate to prevent Pieface from failing Mrs Creecher's maths test! He persuades the gang to use Professor Von Screwtop's mind-swapping device. But in a disastrous turn of events, Screwtop is on the loose with the brain of Paul the potato and Pieface is left with a potato with the brain of Screwtop to help get him through the test!

After Dennis persuades Pieface that Paul is an undercover super hero, Gnasher, JJ and Rubi go along with the story and help Dennis to 'prove it'. However, his genius plan comes to a sticky end, when Pieface begins to think that with Paul at his side, he is completely invincible!