Four friends have a pact: to meet up on the same day every year in their small hometown in southern Italy. Art, the charismatic leader of the group, has always been adamant about the pact. But this year, he doesn't show up.

Searching for him, the friends make a worrying discovery: Art has been farming marijuana, a very dangerous activity in mafia-controlled country. Asking around in town, they hear bizarre and incredible rumours that Art cured a young girl of leukemia.

And then, amongst the chaos of his house, they find a curious manuscript written by Art called The Book of Hidden Things, which promises to reveal secrets enchanting as flames, and just as treacherous...