Our hero in this French animated movie (set in New York City!) is Leo (voiced by Alex Gagnol), an 11 year old being treated at the hospital for some really nasty disease that looks like it's probably cancer. But he has a power: he can float away from his body unseen, fly and explore the world as a phantom. He befriends a cop (Édouard Baer) who's also in hospital, and together they try - with cellphones and Leo's skill - to prevent "The Face" (Jean-Pierre Marielle) from destroying the city.
They use a plain style of slightly shaky 2D animation that I'm not a fan of. But the longer I watched, the less I cared: their only concession to the kids that are the target audience was to make the villain and his henchmen somewhat comedic instead of flat out scary. They show Leo's parents leaving his hospital room having put on a good face for him, breaking down crying - and Leo sees it because he's a phantom. A heart-breaking, charming, and wonderful piece of work.