1907. At the end of the river, something is hungry.

At the height of the rubber boom, a remote tapping station on a black-water tributary of the upper Rio Negro has gone silent. No shipments. No men. Not a single word. The baron who owns it charters a wheezing steamer and a handful of men to push upriver and reopen it — whatever the cost.

Only one person aboard can truly read the water: Mariana Sá, a mixed-blood cartographer the bosses despise and cannot do without. She soon understands that it was neither fever nor rebellion that emptied the station. It was a predator science has never named — an animal driven up from the depths by the barons' dynamite and axes, one that has found a new prey, slow and soft and defenseless: man.

The farther the expedition climbs, the tighter the river closes around them. The banks shut like a jaw. The steamer can no longer turn back. And the beast never sleeps — it waits beneath the black surface, patient, and takes them one by one.

To survive, Mariana will have to do what no man aboard dares to consider: go higher still, all the way to the source where the thing was born — and face it on its own ground.