This story contains spoilers for Happy Death Day 2U.
There are plenty of movies that should never get sequels. Happy Death Day, a runaway sensation in 2017, seemed engineered to be a one-off, a low-budget riff on slasher horror by Blumhouse Pictures that would have made its money back no matter what. Instead, it turned out to be surprisingly popular -- its snappy wit and themes of self-acceptance and forgiveness making it a horror-comedy sensation. The plot -- a girl is murdered and wakes up on the same day over and over until she finds out who her killer is, gradually coming to terms with her mother's death and estrangement from her father in the process -- is perfectly self-contained. So when the movie made a gajillion times its budget, the question wasn't whether it would get a sequel, but how to rejigger the story to advance the action without, you know, ruining everything that made the first film special.
Happy Death Day 2U doesn't exactly improve on a perfect formula, but it expands the narrative just enough to make a case for itself without steamrolling over what made its predecessor so popular. It begins by following a different person: Ryan (Phi Vu), the roommate of Carter (Israel Broussard), the boy whose room Tree (Jessica Rothe) repeatedly woke up in over the course of the first movie. This time, it's Ryan who wakes up over and over on the same day, unable to figure out why it's happening to him.
One change in approach is that the sequel offers a more concrete explanation of the time loops: For a science project, Ryan and his nerdy buddies have created an engine that, when tuned to the right frequency, creates folds and loops in the multiverse, sending our heroes crashing in and out of their own dimensions. Naturally, Ryan and his team turn the engine back on to try to reverse the effects, but instead of freeing Ryan from his unintended loop, it sends Tree back to her birthday, the day of all those murders. This time, though, the machine has shunted her into a parallel universe where everything is just slightly off. The professor she was having an affair with has never met her, the sorority sister who was trying to kill her in the first movie is her best friend, and Carter is dating someone else and barely recognizes Tree at all.