The movie follows Tree Gelbman (Jessica Rothe), a college student who keeps reliving the same day over and over again. Every day is the same: she wakes up to the song "Happy Death Day" and is killed by a masked killer.

In the climax of the movie, Tree finally discovers that the killer is Lori Spengler (Ruby Modine), a seemingly sweet and innocent sorority girl. Lori was a former patient of Dr. Butler who became unstable after a traumatic event.

As Tree tries to solve the mystery, she starts to develop feelings for her professor, Dr. Gregory Butler (Charles Aitken). She also becomes more empathetic towards her friends and starts to care about them.

At first, Tree tries to take advantage of the situation by doing crazy and reckless things, but she soon realizes that she is being killed by a masked killer every day. She starts to investigate her own murder and tries to figure out who the killer is and why they are killing her.