What happened?

For as long as he can remember, Luc has been a sleepwalker, inexplicably drawn to the green hills by his home. And for four years, his sister Cora has been suffering from a mysterious illness that leaves her in perpetual lethargy, despite most of her days spent asleep. But Cora insists that when she closes her eyes, she does not sleep, and is instead trapped inside the green hill of Luc's dreams.