Ever since Congo was handed to Leopold II, the humanitarian king of Belgium in the late 19th century, a trickle of reports on human rights abuse have been coming down from the dense jungles. After recent media attention back home, the British government has asked Roger Casement, their representative in the African free state, to look into these claims. He travels deep into Congo's interior, to the shores of Lake Mantumba, where a local man named Nsala helps him expose the truth while a Belgian official Gaspard Bunschoten tries his best to maintain an illusion that is doomed to fall apart.