"Plump helmet men are known for their clever jokes and games, as well as their innocent, almost childlike nature. This has led to them being universally beloved by all sentient creatures of the world, as well as dwarves, who find them both sweeter and more succulent than the farm-grown variety. For years, dwarves have been trying to find a way to brew these majestic creatures alive, but the scientific effort has been frustrated by the tendency of dwarven brewers to pounce on any plump helmet man they see and eat their way through the still-living myconid in an orgy of mushroom juice and drool.

However, beneath the dumb exterior, they have a darker side. They are secretly geniuses, who strive constantly to escape."
Dwarf Fortress lore on Plump Helmet Men