Will is thirty-six but acts like a teenager. Single, child-free and still feeling cool, 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 (and grateful) mothers, all waiting for Mr Nice Guy. That's where he meets Marcus, the oldest twelve-year-old in the world. Marcus is a bit strange: he listens to Joni Mitchell and Mozart, he 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 can help Will grow up - and they can both start to act their age ...