One day every hundred years, a town appears, its location and character different every time. It is home to the greatest miracle a man could imagine: a doorway to Heaven itself. The town's name is Wormwood, and it is due to appear on the 21st September 1889, somewhere in the American Midwest.

There are many who hope to be there: travelling preacher Obeisance Hicks and his simple messiah, Soldier Joe; Henry and Harmonium Jones and their freak show pack of outlaws; the Brothers of the Order of Ruth and their sponsor Lord Forset (inventor of the Forset Thunderpack and other incendiary modes of personal transport); and finally, an aging gunslinger with a dark history.

They will face dangers both strange and terrible: monstrous animals, predatory towns, armies of mechanical natives, and other things besides. Wormwood defends its secrets, and only the brave and resourceful will survive...