Will is thirty-six acts like a teenager. He reads the right magazines, goes to the right clubs and knows which trainers to wear. He's also discovered a great way to score with women – at single parents' groups, full of available mothers, all waiting for Mr Nice. That's where he meets Marcus, the oldest twelve-year-old in the world. Marcus is a bit strange: he listens to Mozart, looks after his mum and he's never even owned a pair of trainers. Perhaps if Will can teach Marcus how to be a kid, Marcus will help Will grow up. . .