Two men show extraordinary courage by secretly mapping Paris' underground during the 1940 German occupation.

On the mystic island of Lemuria, the cult of Ubasti seek the Egyptian Princess Nadji to sacrifice so that their goddess Ossana, whose soul resides in Nadji's body, may be resurrected by Black Magic. Nadji is located in the Far East port of Suva, but shielded by the White Magical powers of Frank Chandler, an American raised by Eastern mystics who is also known as Chandu. When Chandu takes a voyage alone, however, the evil Voice of Ubasti is able to magically spirit her to Lemuria, where Black Magic reigns supreme. Chandu sets out in pursuit with his sister Dorothy, niece Betty and nephew Bob; but, shipwrecked on the magic island, Chandu finds his family also held prisoner for sacrifice while he is plunged into an endless maze of caverns beneath the evil temple, where both his mortal and magical strength seem rendered useless.

While shooting a documentary on the suspicious disappearances within the homeless community, a filmmaker and his crew go missing while uncovering a terrifying and vicious secret below the city's surface.