Castiel grew up in Heaven, watching the Earth from above with his (rather overprotective) big brother, Gabriel. His dream was to go there himself one day. And his wish very soon comes true, when his father allows him to go.
He meets a young human boy named Dean. He grows quite fond of this boy, and they become fast friends. One day, he sneaks out of Heaven using the one thing that can take him from Heaven to Earth and back again. Tragically, he loses it on Earth, and is stuck there with Dean.
So he grows up on Earth with his best friend, while his family tries to find him. But he soon learns that Dean is more than his best friend, and he also learns that Dean feels the same way toward this angel who lost his wings.