Regina George (Rachel McAdams)’s Timeline and Summary

Regina meets Cady and invites her to eat lunch with The Plastics.
The Plastics show Cady their Burn Book.
Gretchen tells Regina that Cady likes Aaron, and Regina tells Cady she'll put in a good word for her with him.
At the Halloween party, Regina tells Aaron that Cady's a stalker and then comes on to him. They get back together.
Regina flaunts her relationship with Aaron in front of Cady.
Regina wants to lose three pounds.
Regina snaps at Gretchen during the Winter Talent Show, prompting Gretchen to start spilling her secrets to Cady.
"Somebody on the baseball team" tells Aaron that Regina's cheating on him, and they break up.
Cady uses a three-way call attack to turn Karen against Regina, too.
Regina and Shane crash Cady's house party, and Regina walks in on Cady and Aaron talking on Cady's bed. She loses it.
Regina adds a page about herself to the Burn Book and then turns the book into Mr. Duvall in an attempt to frame Cady, Gretchen, and Karen for the book.
Then she distributes copies of it all over the school and chaos erupts.
Janis tells all the junior girls about she and Cady's plan to destroy Regina's life, and Regina leaves school.
In the middle of chewing Cady out in front of the junior girls, Regina gets hit by a bus. She injures her spine, but is ultimately fine.
By senior year, Regina and Cady have repaired their relationship. Regina's also joined the lacrosse team.