NOTE: The following post explains what you need to know about Happy Death Day before you see Happy Death Day 2U. It has SPOILERS for the first movie, but not for the sequel; it’s meant to be read as a refresher or as the cinematic CliffsNotes before you see the new movie.

If you’re going to see Happy Death Day 2U this weekend, it might help to have watched the original movie as many times as Tree lived through her birthday. The sequel to 2017’s Happy Death Day does feature an onscreen recap of the events of the first movie — where a mean sorority sister gets trapped in the day she is murdered by a masked killer — but it’s pretty brief and cursory, and 2U actually relies on your knowledge of the original movie for most of its plot, and a lot of its best jokes. If you haven’t seen Happy Death Day, or you haven’t seen it since it premiered, you’ll at best miss some of the smartest jokes and at worst be kind of lost. So here’s everything you need to know.

Both movies are about Tree (Jessica Rothe). She wakes up on the morning of her birthday in the dorm room of a guy she doesn’t know named Carter (Israel Broussard). That night, someone in a creepy baby mask — the mascot of her school, Bayfield University — kills Tree, and she immediately wakes back up in Carter’s room the previous morning. Tree keeps living the day over and over — and dying over and over — as she tries to find her killer.

Charles Aitken as Gregory Butler and Jessica Rothe as Tree in ‘Happy Death Day’ / Universal

The only way to solve the mystery is process of elimination. The most likely suspects include her married biology professor, Gregory (Charles Aitken), who she’s been carrying on an affair with), and John Tombs (Rob Mello), an ominously named serial killer who escapes from a nearby hospital. This is a horror movie, though, so the obvious suspects are red herrings; the real killer turns out to be Tree’s seemingly friendly roommate Lori (Ruby Modine), who hates her because she was also having an affair with Gregory, but he spurned her to pursue Tree. At the end of the film, after about a dozen tries, Tree kills Lori, hooks up with Carter, and ends the loop.

She never figures out what started the loop in the first place, though, and that’s important to know, because the reason why she got trapped in the first place is a big part of the sequel — where Tree somehow (I won’t say how) winds up back on her birthday again and again and again.

A few other important details to note from Happy Death Day that are important for HDD2U:

-Carter’has a nosy roommate named Ryan (Phi Vu), and Tree has a serious frenemy in her sorority house named Danielle (Rachel Matthews). Both characters had small roles in Happy Death Day; they’re far more important in HDD2U.

Jessica Rothe as Tree and Ruby Modine as Lori in ‘Happy Death Day’ / Universal

-Lori originally tried to poison Tree with a birthday cupcake, but Tree threw it in the trash, which is why she went with Plan B: Dress like a giant murderous baby and stab her to death. As one does.

-Throughout Happy Death Day, the electricity on the Bayfield University campus keeps cutting off momentarily. The characters notice it and talk about it, but no reason for the blackout is ever given ... at least in that movie.

Caleb Spillyards as Tim in ‘Happy Death Day’ / Universal

-Another red herring suspect in the first movie is Tim, who first appears as Tree is walking home from Carter’s dorm room. He follows Tree around campus, and he repeatedly asks her why she isn’t returning his calls. As a seemingly jealous and obviously muscly dude, he would make a perfect murderer — but it’s one big fake out. (Turns out Tim is gay.)

-Last but definitely not least: Tree goes on a big emotional journey in the first film that’s similar to the one that Phil Connors has in Groundhog Day. When Happy Death Day begins, Tree is mean and selfish and she pushes away anyone who cares about her. Eventually we learn a lot of this is a reaction to the death of her mother a few years earlier. After her mom passed away, Tree pushed her father away and became a colder, isolated person. Dying teaches her to live again, as it so often does in this kind of movie. But how this all started is definitely something to keep in mind when you head to the theater for Happy Death Day 2U. Which you should do this weekend! It’s a fun sequel.

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