Screw-On Head is an agent for President Abraham Lincoln. He is summoned by Lincoln to track down Emperor Zombie, an undead occultist and originally a groundskeeper at Hyde Park. Zombie and his henchmen, the vampire Madam and scientist Dr. Snap, have stolen an ancient manuscript. This will allow him access to the temple of Gung, a warlord who nearly conquered the world over ten thousand years ago with supernatural power gained from "a fabulous melon-sized jewel", which Zombie obviously plans to use for himself.
With the aid of his manservant, Mr. Groin, and dog Mr. Dog, Screw-On Head manages to track down Zombie, but not before the villain and his henchmen find the treasure: instead of a jewel, the tomb contains a turnip with "a small parallel universe" inside. Zombie unleashes the Demigod within, but Screw-On Head manages to defeat it in combat.