The original Happy Death Day was Scream through the filter of Groundhog Day. In the first movie in the Blumhouse franchise, Tree (Jessica Rothe), a sorority girl with an attitude problem, finds herself in a time loop: She is murdered, every day, by an anonymous killer in a baby doll mask. The only way to break the cycle, and survive to the next day, is to hunt down the murderer before he hunts her.
That's what Tree does in the first film: She discovers that it's her roommate Lori (Ruby Modine) who has it out for her, and uses a serial killer named John Tombs (Rob Mello) as the perfect scapegoat. Tree kicks Lori out a window, and finds romantic bliss with Carter (Israel Broussard), the boy-next-door type who helped her through this whole mess in the first place.
Happy Death Day 2U is a little bit more complicated. In this movie, Tree wakes up in a time loop once again — but in a different dimension, where things are just slightly off. For one thing, Carter is dating Danielle (Rachel Matthews), Tree's absolutely-the-worst sorority sister, which guts her. But this universe does have its perks: Tree's much-missed mother is alive, which makes Tree desperate to stay in this plane.
Tree and her gang of quantum physics students — Samar (Suraj Sharma), Dre (Sarah Yarkin) Ryan (Phi Vu), whom the first movie wrote off as Carter's obnoxious roommate — attempt to close the time loop so that Tree can remain in this universe. Except, as Carter points out, this isn't Tree's life. Yes, her mom is alive, but losing her mother, as painful as it was, made Tree who she is today.
Eventually, Tree realizes that the memories that she has with her mom aren't even really her memories — they belong to a different version of Tree. She attempts to call and tell the quantum physics scientists to send her back to her old dimension via their "Sissy" invention, but can't get through to them — so, she drives her car directly into the electrical circuit of her university, killing herself and effectively shutting down the change in time loop.
When Tree wakes up (once again in Carter's bed), she makes the decision to say goodbye to her mother, and to right all the things she must before going off to her own dimension. One of these tasks is ensuring that the people who will be killed by the baby doll murderer in the hospital survive to the next day.
So, Tree heads to the hospital where she saves a (much less psychopathic) version of Lori from John Tombs and a mysterious second killer. That killer is none other than doctor-slash-professor Gregory (Charles Aitken), who, apparently, wants to kill Lori (his mistress in this universe) so that their secret affair will die with her. His wife is also in the hospital, because she, too, wants to see Lori dead and buried. Gregory, however, prefers to be a bachelor and kills her, too.
Gregory shoots Lori, then chases Tree down in the hospital to kill her. They end up in a showdown in the MRI room, where Tree uses the machine's super strong magnetic pull to pin Gregory against the machine with a metal wheelchair. She then sends a screwdriver flying into his stomach, killing him.
It's moments later that the quantum physics squad finally sends Tree back to her "correct" dimension — though not before Tree and Carter share an epic kiss. All's well that ends well, right?
Err... maybe not for Danielle. In a post-credits scene, government officials head to the lab where they explain they'll need to borrow "Sissy," the machine Ryan has developed for his thesis project. They want to experiment with time loops, too — and need someone to experiment on. Tree suggests that Danielle — who was cheating on Carter in the previous dimension — would be the perfect test subject. Cut to Danielle waking up and screaming... guess it's someone's not-so-happy death day again.