We've made it through the first act to that second window of opportunity. What happens here needs to do two things:
1) It needs to set up the final conclict. We've developed characters and built tension between them, but people like to avoid confrontation. In mysteries and thrillers, the villain especially likes to avoid it. If your book is a deeply introspective novel, you might find that your characters would rather not confront each other. We all have put up with people we dislike for long periods. Maybe the person who gets under your skin is a hypocritical scout leader or your best friend's wife. You can avoid conflict with that person indefinitely--still, privately fuming at their shortcomings.
The second window has to be an event that forces your Lead into a confrontation that will resolve your central plot problem--and maybe completes a character arc too.
2) What I want to emphasie to you is that this window should be made of one-way glass. Climbing through it needs to create a situation where you can't see what's going to happen, and once you're through it, there's no going back. You have to face the confrontation.
If your readers care about your Lead, they've just watched him suffer at the midpoint, then they will be eager to follow this through to the end. They want to see things turn around for him. And maybe his luck won't change, but they can't help reading on--to find out.
So write a one-way window of no return.
