This thriller begins when a herb called Chalawan is sent to a fourth-year student of the Botany faculty, Nawa. Wanting to know the source of the rare plant, Nawa invites his friends Burin, Namwhan, Champagne, Fifa, et al. to travel through the western forest to meet with his relative Phi Sak who discovered the Chalawan plant. They meet with the village leader Ongsu at the lagoon deep in the forest who tells them Phi Sak has already left. He also tells them that the Chalawan herb they're looking for is on an island far away. Importantly, gathering the herb can only be attempted at twilight. As the journey of finding the Chalawan plant progresses, so too do the past relationships of each member. All the while, the darkness creeps up on them and they discover they're in danger of falling prey to an ancient crocodile.