What are two main events in the story?

Four at last reveals to Tris his true identity: he is Tobias Eaton, son of abusive Abnegation council representative Marcus. At this point, they officially begin a secret relationship.

Tris realizes the only way to stop the killing is to stop the simulation entirely, so she, her father, her brother, and Marcus Eaton head to the Dauntless control room to shut it down.