Because Twins is a children's book, however, it is customarily omitted from listings of the main Tarzan series.

Invited to visit Tarzan's African estate, they become lost in the jungle and are imprisoned by cannibals, from whom they escape.

The brutish Oparians, adherents of the deposed and deceased high priest Cadj, are seeking a place to continue worship of the Flaming God in the traditional way, as Opar's Queen La has outlawed human sacrifice in the lost city.

They plan to make their young captive Gretchen, whom they dub "Kla", meaning "New La", their new high priestess.

She is eventually rescued by Tarzan and the twins, while Glum, leader of the exiles, and some of his adherents are killed.