The Sub-Heroes, a team of superpowered fourth graders, have already saved the city once. Since that day, they've been training harder than ever to be ready for the day another supervillain rises. That day has come, and they are not ready. Investigating a robbery at a local school, the team learns the theft was part of something much more sinister. Even their incredible powers may not be enough to battle the entire team of supervillains known as The Nightwatch. All hope may already be lost. They are Sub-Heroes. They must succeed. The entire city depends on it.