All hope is lost, soon the Big Bad will achieve his/her goal of taking over/destroying the city/world/universe and there's nothing the heroes can do to stop them. Then WHAM! Something unexpected happens, and the tide turns in the heroes' favor, allowing them to overcome the villain and win.
This is what we call a Near Villain Victory, where the bad guys look like they're going to win only to get [...] [tvtropes.org]