Tarzan Lex Barker and Jane Vanessa Brown are spending time by a river when a scream cuts through the air. A local tribal girl has vanished, and the surrounding community suspects an evil spirit at work. The pair quickly realizes the girl was kidnapped, and they uncover that the captors are the Lionians—a hidden, lion-worshiping people with a culture echoing ancient Egypt, living deep in the jungle. These abductors are sweeping the region for girls to bring back to their city, and they carry a deadly disease that can kill within hours.
To fight this threat, Tarzan seeks help from Dr. Campbell Arthur Shields, who possesses a serum capable of both curing the disease and vaccinating against it. After saving the local tribe, Tarzan and Campbell press on toward the Lionian city, joined by Neil Robert Alda, a rough-edged big-game hunter with a drinking habit who provides stubborn but loyal support.
Meanwhile, Campbell’s native assistant Lola Denise Darcel has fallen for Tarzan, adding a layer of romantic tension to the peril. Jane and Lola clash in a cat-and-mouse moment, but their disagreement ends with both being captured by a Lionian raiding party as they push toward the capital.
Beyond the riverine crossings and tribal skirmishes, Tarzan and his companions are repeatedly ambushed by rival tribes and by Lionian forces as they hunt for Lionia. Neil suffers a painful leg injury and is left behind for a time, while Campbell accidentally drops the serum bottle; the crucial vial is later recovered by Neil as he trails the trio.
In the Lionian capital, the situation grows darker. The Lionian king has recently died of the same terrible illness, leaving the Prince in charge. The Prince is easily swayed by Sengo Anthony Caruso, a ruthless counselor who exploits the prince’s grief to indulge his own desires for food, drink, and women. The disease has already decimated many Lionian women, and the men begin to seize local beauties as concubines. When the High Priest challenges Sengo, the schemer convinces the Prince that the priest is a rebel and should be fed to the lions, then takes on the duties of the High Priest himself.
The Prince admires Lola but leaves to check on his ailing son. Lola taunts Sengo with threats of sovereignty, and violence erupts: she’s whipped, and Jane stabs Sengo in the arm with his own knife. The two women flee into the dead Queen’s tomb, a mausoleum carved into the old king’s stone tomb. There, Sengo discovers them and entombs them alive.
Tarzan arrives at Lionia with Campbell just as the Prince’s son falls gravely ill. Sengo blames Tarzan and Neil for the crisis, and orders their deaths. Tarzan escapes, leading a spirited chase through the city. He hides inside the dead king’s sarcophagus, becoming entombed in the mausoleum himself. Yet hope remains: Tarzan soon locates Jane and Lola, who have also been sealed away, and he frees them. Neil appears with the serum, which Cheetah (Tarzan’s animal companion) finds along the way, and they begin to treat the prince’s sick son.
A dramatic rescue follows as Tarzan summons help and an elephant tears open the tomb door, freeing Tarzan, Jane, and Lola. Tarzan holds off the Lionians long enough to hurl Sengo into the lions’ pit. With the prince’s son cured and the danger fading, the Prince recognizes his errors and orders the High Priest, Tarzan, all of Tarzan’s friends, and the captive women to be freed, bringing a hard-won sense of justice and restoration to Lionia.
