Tarzan leads an expedition into the Ras Dashani mountains to find out whether Doris Ramsay's father is still alive. The trek takes them near the Tower of Hotoombo, reported to be the home of a Devil Ogre and guarded by white gorillas.; Tarzan and his friend Paul D'Arnot find Professor James Barton and his daughter Ruth alone in the jungle after their bearers had deserted them. The quartet is attacked by hostile natives and have to flee by canoe. A swift and unexplored river sends the canoe through a long rock tunnel and when they emerge they find themselves in the land of Queen Ahtea and the lost city of Tohr. The Queen decides that Tarzan would make a good husband and King. When Tarzan does not agree with her plans, he and his friends must escape from the Queen's dungeons.; Tarzan and his friend Muviro battle the Arab slave trader Sidi Ben Yemlik, known as the Black Panther.