10 Horror Movie Answers Fans Had To Wait For

In this Groundhog Day-inspired horror, college student Tree Gelbman (Jessica Rothe) is murdered by the masked killer Babyface. She suddenly wakes up, only to learn she's trapped in a time-loop, reliving the day she dies over and over. To stop herself from being endlessly slaughtered, Tree does everything in her power to learn her killer's identity.

Even though it isn't explained how Tree found herself in her timey-wimey predicament, it doesn't stop Happy Death Day from being a total blast due to the creative concept, hilarious dialogue, and Jessica Rothe's career-defining performance. But in Happy Death Day 2U, it's revealed straightaway that this space-time paradox was triggered by a quantum reactor on campus. (Every college has got one of those, right?)

Although this explanation doesn't seem necessary at first, the writers have a lot of fun with it. When Tree tries to stop the reactor from being activated, she finds herself reliving the same day in another dimension. Since this parallel universe has everything Tree ever wanted, she's torn whether to return to her reality or not.

The source behind the time-loop was appreciated, since it put a creative sci-fi spin on the story and stopped the sequel from feeling like a rehash of the original.