Months after his brother Clyde's death at Joe West's hands, Weather Wizard Mark Mardon breaks out of Iron Heights during an unnatural storm, driven by his dead brother's ghost to grow powerful enough to level Central City. As Mark hunts Joe West and the Flash, the storm's strange energy destabilizes Caitlin Snow, threatening to unleash Killer Frost while Cisco scrambles to hold the team together.
Summary by Loreful, from public sources