The Black Magic cult of Ubasti, based on the isle of Lemuria, believes that Nadji, a princess of Egypt, is a reincarnation of their long-dead goddess, Ossana, and intend to sacrifice Nadji so that Ossana may be resurrected. Nadji has taken refuge at the California home of Frank Chandler, an American raised in the east and possessed of White Magical powers, who calls himself "Chandu". Vindhyan, high priest of the cult's California outpost, learns of this and ultimately succeeds in placing her in a trance which Chandu cannot easily break, propelling him to move her to safety, choosing the port of Suva in the South Seas. There, aided by his sister Dorothy, nephew Bob and niece Betty, Chandu is able to revive her and deal with Vindhyan, only to have the evil Voice of Ubasti, highest of the high priests, spirit her to Lemuria through the magic Circle of Ola. Chandu, Dorothy, Bob and Betty set out in pursuit but end up shipwrecked on Lemuria itself, with his family also now held for sacrifice while Chandu is plunged into the endless maze of caverns beneath the evil Temple of Ubasti, shrouded from his own powers by the full forces of Black Magic...