It's the holiday season, but the celebration at the Zeroes' underground nightclub is blown apart when a pair of strangers with new powers take to the dance floor. The Zeroes pursue and confront the two, only to learn that they're fleeing an even more sinister power, the Swarm, who uses a crowd's rage to kill – and who loves killing Zeroes.
The Zeroes defend themselves the best they can, learning more about their enemies through a series of crowd disasters – a wrecked celebrity wedding, a bloodbath at a shopping mall, and a funeral for one of the Swarm's victims that goes very, very wrong. In the meantime, each of the Zeroes has their own fish to fry. Bellwether's confidence is challenged by the Swarm, while Mob finds herself drawn to the killing crowd. Crash's conscience – and her heart – get a workout, and Anon and Scam face harsh truths about fmaily and belonging. Flicker must pick up the groups fallen reins and lead the Zeroes into a final showdown with Swarm's army.