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Happy Death Day 2U's ending explained

Tree Gelbman is living the same day again — again. Horror hit factory Blumhouse has returned to theaters with a mindbending sequel to Happy Death Day, its surprise success story from 2017, and this time, everything's different. The sequel, Happy Death Day 2U, is taking audiences back to Bayfield to finish what the first movie started — or at least build on it. 

If you're confused about some of the movie's weirder plot points, have no fear — we've diagrammed out the timelines and sorted through the sordid details with a summary you can follow, no matter which universe you're watching from. The plot is weird and winding, throwing time-twisting turns and alternate dimensions at you faster than you might be able to process. But while this follow-up may be a little headier than the movie that came before it, there's no reason to be caught out of the loop. Let's break down the ending of Happy Death Day 2U.

Let's recap

The first Happy Death Day follows Tree's journey from a stuck-up sorority mean girl to a sympathetic, gun-toting survivor — a satisfying character arc that felt like Groundhog Day by way of Friday the 13th. In that movie, much like in Groundhog Day, the mechanism that causes Tree to go into a day-repeating time loop is never revealed, appearing simply as a mystical phenomenon that serves to drive the story. But since the movie's release, director Christopher Landon has promised there's an explanation to the phenomenon hidden in the movie, all set to be explored in a possible sequel. 

"The answer to why she's literally stuck in a time loop — it's something I have the answer to," he told Insider in 2017. He also encouraged viewers to look for clues about the loop's origins in the movie itself, saying, "The whole idea for my sequel is actually already in this movie. It's hiding in plain sight."

This assurance from Landon (who has sole writing credit on the sequel, picking up the torch from original movie writer Scott Lobdell), along with the emphasis on Tree's dead mother and a long-lost birthday tradition, led to theories about her mother's spirit having something to do with the loop, with Tree's mother somehow manipulating time from beyond the grave to keep her daughter safe. But Happy Death Day 2U debunks those theories, revealing a source for the phenomenon that's more sci-fi in nature than anyone saw coming.

A Sisyphean task

In what feels like a wild departure from the original Happy Death Day, Happy Death Day 2U reveals that Tree's time-twisting journey was caused by a literal time machine, located in a science lab on the Bayfield University campus. The device, somewhat surprisingly, is the invention of Carter's roommate Ryan, the oafish goober from the first movie with a tendency of barging in every time Tree wakes up.

While the first movie paints Ryan as awkward, boorish, and sex-obsessed, the sequel reveals hidden depths and intellect that would do good by Doc Brown himself. As the opening of the sequel shows us, Ryan is in the process of developing the machine with a team of fellow student scientists named Samar and Sarah. While they haven't perfected the device just yet — and lord knows how they're funding this thing — the team soon discovers that it's capable of disturbing the fabric of space-time, unintentionally sending Tree on her most bogus birthday journey. (Tree is decidedly not happy with Ryan upon discovering this information.)

The lab students who developed the machine call it the Sisyphus quantum cooling reactor, or "Sissy" for short. The name is an on-the-nose reference to the Greek legend of Sisyphus, a man punished by the gods to spend each day eternally rolling a boulder up a hill, doomed to crumple under its weight while never reaching the top. As viewers of the first Happy Death Day know, it's a trial that Tree can quite viscerally relate to.

Isn't it romantic

The introduction of Sissy marks a clear pivot from the first movie's horror genre trappings toward straight-up science fiction, with the plot-explaining time machine having little in common with the more traditionally frightening tone of the first movie. But Happy Death Day 2U also leans into a more romantic angle than its acerbic slasher predecessor, becoming more of a sci-fi rom-com than the Scream-like original. 

When the machine as activated to kick off the movie's second act, it sends Tree into a parallel dimension, in which her relationships with the people around her are ever so slightly askew. The changes are small but significant, forcing Tree to reevaluate her own life and what she wants out of it. 

While the first dimension's Carter is single until he meets Tree, the second dimension version is happily dating Danielle, a rival of Tree's and the alpha dog of her sorority. In Tree's world, her mom is dead — in the second world, she's still alive. As a result of these character change-ups, the tension this time around isn't so much "will Tree survive this nightmare" as it is "will love prevail," with the heroine's biggest challenge being in figuring out whether she wants to live in a reality with her still-alive mother, or her original dimension in which she'd just begun what felt like it could be a loving, long-term relationship with Carter.

Back to the past

Much like ScreamCabin in the Woods, and other winking works of meta-fiction, the characters in Happy Death Day 2U are cognizant of the fact that they seem to be living through the plot of a movie. In this case, parallels are drawn with Back to the Future Part II, the super-fun 1989 movie that saw science student Marty McFly not just revisiting the past, but also the plot of the previous movie, intersecting with the older narrative in new and unexpected ways. 

On top of the Back to the Future sequel being surprisingly relevant to this movie's plot, Happy Death Day 2U also sports a few cute references to the sci-fi series, such as the "Biff's Tree Removal" wood chipper that Tree cheerfully kills herself in at the end of one loop, as well as the fateful speed of 88 mph she attains during a later desperate death behind the wheel.

Quantum mechanics

While not necessarily hard to follow, Happy Death Day 2U clearly doesn't want audiences to linger too much on the how of its storytelling, breezing through plot points faster than you can say "wait, what?" 

After being shot through a hole in space-time to begin the September 18 time loop in a whole other dimension, Tree is faced with two tasks. The first goal is to return to her own reality, a course of action which she can't immediately decide if she even wants to take, considering that her beloved mother is still alive in this new world. But the second goal of stopping the time loop is something she has to achieve no matter what, regardless of which reality she decides to live out her days in. 

The problem is that Sissy is a work in progress, a powerful machine that neither Ryan nor his lab mates yet entirely understand. In order to get it under their control, they need to do more research — lots of research. Each time the loop resets, they lose their progress, with the memories of everyone but Tree being reset to the state they were in when the loop started. As a result, Tree is forced to become the team's institutional memory, keeping track of the complex quantum mechanics that make the machine work. It's a tall task and a battle of inches, with the team only making progress thanks to Tree resetting the loop via offing herself, elaborately and hilariously, over and over again.

The three killers

Complicating all of this multi-dimensional quantum confusion is the inconvenient fact that, just like the first movie, there's a killer on the loose. In the first film, the Babyface masked killer is portrayed by two characters — Tree's sorority sister Lori, and a serial killer named John Tombs, whom Lori manipulates, collaborates with, and sets up to frame for Tree's murder, with her capacity as a nursing student giving her close proximity to the hospitalized criminal. 

In the first movie's reality, Lori was motivated to kill Tree out of jealousy due to her romantic relationship with Gregory Butler, a married professor and medical doctor whom Lori coveted. The doe-eyed nice girl was a most unexpected villain, making for a satisfying twist when she's actually innocent in the second world. So who's killing people in 2U?

The sequel features three characters wielding knives behind the Baby mask. For the most part, the killings in the second world are being committed by Dr. Butler himself, as well as — shockingly — his wife Stephanie, who seems to have been driven insane by Gregory's infidelity. (While they appear to be in cahoots at first, that doesn't really hold up after Gregory decides to shoot Stephanie during the movie's climax. They're apparently just both crazy, and totally deserve each other; what a shame that it doesn't work out.)

The third killer only appears in the movie's first act, while Ryan is experiencing his own time loop on September 19th — and the reveal is a weird one.

The second Ryan

One of the weirdest threads in Happy Death Day 2U is the idea that the Babyface killer pursuing Ryan in the movie's opening is actually another version of himself, invading Ryan Prime's reality in an effort to stop him from causing the time loops in the first place. It's an electric, tantalizing reveal, promising doppelgängers, evil twins, and intertwining timelines that the movie… just doesn't follow up on. Seriously. If you went to the bathroom at some point and thought you missed something, you didn't. You're not crazy — the origins of this second Ryan are never explored, and the mystery of his motivations just isn't answered. 

As John Orquiola with Screen Rant points out, this second Ryan seems to be from the future, possessing knowledge about the time loop's effects that no one beyond Tree should really have yet. 

As much as it seems like it could be a setup for Tree's own doppelgänger being the killer in dimension two, Tree is explicitly told that there aren't two of her in dimension two. When Tree goes to the second universe, she replaces herself in it, and despite the clear establishment of two alternate realities, an alternate version of herself never shows up. So how did Ryan end up with two of himself running around in dimension one? There are many possibilities — but in the movie itself, no answers.

Here come the men in black

Happy Death Day 2U seems to end on the cheerful note of Tree returning to her own reality, the time loop broken, and all of her hard-earned progress in life restored. But the world of the movie gets blown wide open in an out-of-left-field mid-credits sequence, which takes the sci-fi trappings of the sequel and goes all the way with them. 

The scene begins with Tree and her cohorts doing campus clean-up as recompense for their destructive, insubordinate, reality-shattering lab work, the destructive effects of which the faculty can only begin to imagine. But just because the dean and company are in the dark doesn't mean everyone is. Out of nowhere, the students are approached by a whole fleet of dark-suited special agent types, pulling up with stern expressions and a serious agenda. 

A man introduces himself as Dr. Issac Parker with DARPA, an acronym for the real-life, federally-run Defense Advanced Research Projects Agency. The G-People promptly transport the students to their agency's headquarters, where they've set up the confiscated Sissy machine to suit their own purposes. In what seems like a fantastic idea with no possible downsides whatsoever, Dr. Parker reveals that the agency wants to do research with the machine, and requires the students' know-how to make it work. They also need a suitable test subject for further experimentation, proposing a sadistic experiment that no one deserves to experience — except maybe Danielle, who ends the movie waking up and screaming, caught in her own time loop. 

Happy Death Day... Tree?

The ending of Happy Death Day 2U marks a bonkers tonal shift that the sequel has spent its whole runtime building toward, setting up a third movie in a way few could have anticipated. We're not just theory-crafting, either — writer-director Christopher Landon and producer Jason Blum have already said they would like to make a third movie, with the likelihood of it happening all depending on the box office performance of 2U. 

As Blum explained in an interview with CinePOP, the genre-twisting nature of 2U and its stakes-raising ending aren't unintentional. 

"What's different about Happy Death Day," Blum said, "Is I've never seen a franchise where one movie is a certain genre and the next movie is a different genre. And that's certainly the case with Happy Death Day and Happy Death Day 2U, and hopefully if we make a third one it'll be a different, third kind of genre."

As disappointing as it might be on some level to leave behind the slasher tone, it's hard not to be interested in where this story could go. From the sound of the filmmakers' comments, the sequel's already abandoned horror quite deliberately. So if the audience turns out for it, why not abandon everything, and just go full-bore nutso in the next one? Could this movie be designed with loose ends to be tied up later, such as the mysterious second Ryan? It certainly seems that way — and if the threequel gets the greenlight, we can't wait to see where the story goes.