Marvin loves going on his adventures with his grandad – escaping from hairy yetis, taking day trips to Mars, and hunting ferocious tigers – all without leaving the house. Marvin thinks his grandad is the bravest person he’s ever met; he even has a medal from the war to prove it. And, more than anything else, Marvin wants to be brave too.

But when Grandad must go on his final adventure alone, Marvin finds he has to be braver than he’s ever been before. Until he discovers that Grandad has left behind a very special surprise just for him…