Nam Chul-woo is a poor fisherman living a simple but happy life with his wife and daughter on the north side of a river that divides the two Koreas. Every day he goes fishing on the river, where the checkpoint soldiers know him well and trust him not to cross the invisible border in the water. But one day his fishing net gets caught in the boat's engine, and Nam cannot stop himself from drifting into the South. Upon reaching shore, he is immediately seized by South Korean border police and thrust into a brutal investigation, whose ambitious interrogating officer begins grooming him to be a defector and a spy. As Nam's ordeal intensifies, it becomes apparent that, even should he manage to return home, his life in the North will never be the same.