Mignola got the idea from action figures, particularly Batman ones, which seemed to be the exact same figurines with different clothes-and heads. So he imagined a robot with a head that screwed onto different bodies to suit the occasion - hence the "Screw-on Head".
The Screw-On Head is an agent for President Abraham Lincoln, fulfilling much the same role as the modern-day fictional B.P.R.D. (Bureau for Paranormal Research and Defense).
In the comic (and television pilot), 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 vampiress "Madam" and scientist Dr. Snap, have stolen an ancient manuscript. This will allow him access to the tomb 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 (including his vampire lover) find the treasure: instead of a jewel, the tomb contains a turnip with "a small parallel universe" inside. Zombie unleashes the demon within, but Screw-On Head manages to defeat it in combat.(wikipedia.com)
Recommended to me by my friend and yours, The Fraze. I enjoyed it. Extremely short and cut together like carrots on the cutting board with it's quick fast paced slices adult cartoonic television.
Voiced by Paul Giamatti and David Hyde Pierce the adventure is one of ridiculous proportions. The Amazing Screw-On Head was a delicious 22 minute snack before bed.