Warning: This article contains major SPOILERS for Happy Death Day, and minor SPOILERS for Happy Death Day 2U.
Comedic horror sequel Happy Death Day before heading to the theater. Perhaps more than any other genre, if a horror film is successful at the box office, it'll probably get a sequel. In fact, it might spawn a whole franchise. The relatively low production costs of most horror films have a lot to do with this, as does the overall loyalty of the horror fanbase. However, in many cases, horror franchise entries are loosely connected to each other, carrying over only the premise and/or recognizable villain.
For example, the iconic an absolute mess when it comes to continuity. That said, those assuming they can just go into Happy Death Day 2U without having seen the original are quite mistaken, as knowledge of the first film is necessary to understanding where the sequel takes things.
Related: Does Happy Death Day 2 Have A Post-Credits Scene (& How Many)?
In fact, even those who saw Happy Death Day when it was first released in 2017 are still urged to watch the film again before seeing the sequel, as Happy Death Day 2U jumps straight back into the story with only a quick and cursory recap provided by Tree (Jessica Rothe) herself to fill in any blanks. As alluded to in the sequel itself, Happy Death Day 2U takes inspiration from Tree back into an altered version of her original time loop. So many of the plot turns, funny gags, and character beats won't make sense without knowledge of the first film fresh in one's mind.
Without giving too much away about Happy Death Day 2U, Tree's return to the beginning of her Happy Death Day time loop - albeit an altered version - allows some of her relationships with other characters to be reexamined in new, interesting lights. This includes her morally dubious friend Danielle, her burgeoning love interest Carter, and her roommate Lori, who fans will recall turned out to be Tree's real killer in Happy Death Day's ending. Those unfamiliar with Tree's prior dynamics with these characters won't fully absorb why what's changed is such a big deal. Some things about Tree's own situation change as well, and without seeing the first film, viewers won't be able to appreciate the emotional consequences this has for her.
Additionally, the first Happy Death Day movie establishes that Tree's various deaths take a toll on her, physically, with her feeling her injuries the "following" day. This continues in Happy Death Day 2U, but it isn't really explained; the sequel basically assumes people are aware of the previous movie's story. Tree is also shown to be having an affair with a married college professor in the first Happy Death Day, a relatively minor storyline that gets a new significance placed on it in the sequel. Finally, Happy Death Day 2U is full of hilarious moments that directly call back to similar moments from Happy Death Day, sometimes in a subtle way. To pick up on and enjoy them all, a recent viewing of the original is absolutely essential. Happy Death Day 2U's plot might not as convoluted as Happy Death Day 3 ever comes out, fans should expect to need to catch up yet again.
