Waxillium Ladrian, a frontier lawman turned big-city senator, has been on the hunt for the Set, a clandestine organization led by his late uncle and sister, since they began kidnapping people with the ability of Allomancy in their veins. When Detective Marasi Colms and her colleague, Wayne, discover stored weaponry heading for the Outer City of Bilming, it opens up a new avenue of investigation. The war between Elendel and the Outer Cities only benefits the Set, and their tendrils have now reached the Elendel Senate—whose corruption Wax and his wife, Steris, have worked to expose—and Bilming has become even more entwined.
After Wax discovers a new sort of explosive capable of unleashing immense havoc and believes that the Set must already have it, an immortal kandra serving Scadrial's god, Harmony, reveals that Bilming has been influenced by another god: Trell, who is worshiped by the Set. But Trell isn't the only Cosmere force at work. Marasi is being recruited by people from other worlds with strange skills who say they want to protect Scadrial no matter what.
Wax must decide whether to put aside his tumultuous relationship with God and become the Sword that Harmony has prepared him to be. If no one steps forward to be the hero Scadrial requires, the planet and its millions of people will be destroyed in an instant.