Summary (Part 1)
Tantor returns from the nearby river with some sort of poison. After analyzing it, Professor Porter concludes that it must have come from the river, and so Tarzan and his friends convince Dumont to allow them to borrow his boat, as the poison will affect trade and jungle life. Terk stays behind with Tantor, while the others travel upriver, only to have their boat blown up by pressure caused by hippos. Going ahead, Tarzan encounters Basuli, son of Chief Keewazi of the Waziri tribe, who say that men dressed like Jane and the Professor dug in the mountainside. Meanwhile, Tantor has somehow gotten better and with Terk goes after his friends; Basuli shows Tarzan a mysterious cave shaped like a skull head.
Summary (Part 2)
Tarzan and Basuli, upon further exploration, discover that the cave leads to a mine under the direction of a man named McTeague; Jane and Professor Porter meanwhile inform Keewazi that due to the chemical separation of dirt from ores, the chemicals are being dumped into the river. Tarzan and Basuli infiltrate the mine, but are captured by McTeague. With some help from latecomers Terk and Tantor, Jane and Professor Porter rescue the pair, who have become friends through their trials together. Jane then proposes they build a dam, to cut off the water flow. When McTeague sees this and demands to have the water back, Keewazi unleashes the river all at once upon the mine, destroying it. After this, Tarzan makes peace with the Waziris.