For a three-act plot analysis, put on your screenwriter’s hat. Moviemakers know the formula well: at the end of Act One, the main character is drawn in completely to a conflict. During Act Two, she is farthest away from her goals. At the end of Act Three, the story is resolved.
Act I
Verloc gets his mission from Mr. Vladimir—bomb the Greenwich Observatory—and then spends the next chapter stressing about what he's going to do. After listening to his anarchist "friends" wax philosophical about capitalism for a while, he realizes that not one of them has the wherewithal to plant a bomb for him.
Act II
We learn from Comrade Ossipon that a bomb has gone off in Greenwich Park, and someone has blown himself to pieces. This sets off a police investigation that involves Chief Inspector Heat tracking the crime back to Verloc's shop, while the Assistant Commissioner of Police takes it upon himself to beat Heat to Verloc and wrap up the case personally.
Act III
We discover that Verloc has used his mentally disabled brother-in-law to commit the outrage at Greenwich. He gave Stevie the bomb to plant, but Stevie tripped on his way to the observatory and blew himself up. Winnie finds out and murders Verloc. After Ossipon betrays her, she then commits suicide. We're left with Ossipon and the Professor drinking together, neither one of them feeling particularly good about his place in the world.