Tara is captured by the Reptons, who place her in a mind control device to make her their evil queen. She orders her people to destroy Zandor and the Herculoids, who beat them and go to rescue Tara. The Reptons send the Destructo Bats after them, but they escape, recover Tara, free her from the machine's influence, and destroy the Reptons' underground lair. While investigating panic among the Monkey People, Zandor confronts a giant spider and is bitten and poisoned. Dorno must go to the six-legged mustachioed Spider People within the Endless Caves with the Herculoids to get the only known antidote. The Spider People capture Dorno to hold him hostage. The Herculoids eventually stage a rescue, get the antidote, and block the Spider People's caves to cover their escape.