Wax, Wayne, Marasi, and Steris are tasked with finding the Bands of Mourning. They are said to contain the Lord Ruler's powers. Over the course of the book we find out that 1) The Set has been playing with Hemalurgic spikes to give themselves Allomancer's powers and 2) There is an entire civilization to the south that has advanced technology compared to Wax's people. The Set had captured an airship of the other people and tortured the location of the Bands of Mourning from them.
Wax ended up getting to the Bands of Mourning first. There was a temple with pretend bands that didn't really work, but Wayne found the real one when he stole the spear at the entrance. With the Bands they were able to defeat the Set's soldiers that were there, capture Wax's uncle, and hurt Wax's sister who was another leader in the Set. Wax's government agreed to start talks with the southern people to start trade with them.