The narrator meets the Martians, who turn out to be not your loveable type of extraterrestrial. (No Reese's Pieces for them.)
When it comes to The War of the Worlds, we could break the story down in a few ways, but here's one way that makes sense to us: up to around Book 1, Chapter 9 ("The Fighting Begins"), the narrator isn't totally caught up in the action. That is, there are Martians around, but they remain stuck in their crater, so the narrator can still go home and pretend to live an ordinary life.
It's only when the Martians come out of their crater that the narrator reaches "the point of no return" – which is how we know it's the end of Act I.
The Martians terrorize the countryside and the narrator ends trapped in the house with the curate.
Here's how we separate Act I and Act II: at the end of Act I, the narrator starts running around, and he keeps running (and hiding) until the end of Act II, when he can't run anymore because he's trapped in the ruined house.
Now, if you thought the narrator shouldn't keep running around (because, you know, there are Martians around who could catch him), you might think being trapped in the house would be a good thing. But we could also say that this prevents him from reaching his ultimate goal. He really wants to run and find his wife, but instead, he's stuck playing house with the crazy curate.
The narrator finds the world changed – but at least the Martians are dead.
In your average movie, Act III is where the protagonist finds the strength to go on from the low point of Act II – the romance works out, the aliens are defeated, the underdog wins the race, etc. However, in The War of the Worlds, Act III is all about the consequences. The Martians defeated the humans, so the countryside is covered in red weeds and is generally ruined.
The Martians didn't think to bring along hand sanitizer (and they have no immune system), so they die of simple infections. In the final chapter, people have experienced this crazy alien invasion, so they are forced to reconsider their place in the universe. And, in the narrator's case, he may suffer from some Post-Traumatic Stress Disorder.