This sequel to 'Happy Death Day' opens with Ryan (Phi Vu) waking up in his car after being forced to sleep there. He goes to his room to find Tree (Jessica Rothe) and Carter (Israel Broussard) hooking up, and Carter makes him get lost. Ryan goes to meet with his friends Samar (Suraj Sharma) and Dre (Sarah Yarkin) in the school's science lab. They have been working on an experiment involving a reactor they have built and are on the verge of a breakthrough, until Dean Bronson (Steve Zissis) comes in and orders them to shut their project down due to the constant damages it does to the school's power.
Later, Ryan goes through the science lab in the dark and checks the closet. He is startled by Samar appearing behind him, only for the Babyface Killer to emerge from the closet and stab Ryan in the chest. Ryan then immediately wakes up at the beginning of the same day, and he notices everything happens the same way it did the first time. He goes to his room to interrupt Tree and Carter to tell him what he just experienced. Tree figures he is now caught in his own death time loop and recaps the events of the first movie to him to get a gist of what has gone on. She and Carter agree to help Ryan find out who's trying to kill him. They retrace his steps back to the lab, but nobody is in the closet when they look there. When Ryan shows Tree and Carter the reactor, she realizes Ryan was the one who created the time loop that caused her to keep repeating Monday the 18th.
Tree and Carter are at a pep rally when a fire alarm goes off. As the students are being escorted out, Ryan spots Babyface staring right at him and starts to run in the opposite direction, with Babyface following closely. He catches up to Ryan, but Tree knocks him out with a bat. They take off his mask and are shocked to find that the killer is none other than...RYAN. Actually, it's Ryan from an alternate time line. He has come to kill the Ryan from this time line to prevent him from creating a catastrophic event with the reactor. Ryan 1 goes to activate the reactor to try and fix the time lines, just as Bronson arrives to try and stop them. Ryan 2 attempts to destroy the reactor, but it goes off and causes a crazy blast that knocks everyone back.
Tree wakes up in Carter's room. He repeats the same things to her that he said every time she woke up on her birthday. She hears her birthday ringtone and realizes she has once again found herself waking up on Monday the 18th. Tree freaks out and angrily storms the campus with Carter and Ryan to the lab to figure out what's going on. They get Samar and Dre together after discovering that their reactor works, but Tree demands that they fix it so she can return to a normal day.
Tree goes to her room to find Lori (Ruby Modine). She asks for the poisoned cupcake that she figures Lori was preparing, but she tells Tree there is no cupcake, which means that Lori is not planning on killing Tree. While in the dining hall, Tree sees in horror that, in this time line, Carter is dating Danielle (Rachel Matthews), who is apparently not rotten like the Danielle that Tree knows too well. Tree then goes to join her dad David (Jason Bayle) for their birthday lunch. To Tree's very big surprise, her mother Julie (Missy Yager) is alive in this dimension. She goes to tearfully embrace her mother for the first time in years, and the three spend their time celebrating both Tree's and Julie's birthdays. Tree later looks through her phone and sees years of memories with her mom that she never knew about.
Just as Tree is deciding to stay in this dimension, she sees a news report about John Tombs (Rob Mello) being taken to the hospital. She rushes there to kill him and grabs a fire ax. As Tree walks in to Tombs's room, she is caught by a cop and is forced to drop the ax as he cuffs her. The cop is escorting her out when Babyface stabs the killer to death. She still thinks it's Lori until Lori turns up in the elevator behind her and grabs Tree to safety. Lori takes her through an area that's under construction, but Babyface gets them there and stabs Lori in the chest, killing her. He then chases Tree up the stairs, but she manages to trip him down. He gets up quickly and catches Tree on the roof. She loses her balance and falls to her death.
Tree wakes up on the same day again and sees she is still in the alternate dimension. She goes with Ryan to the lab to explain to him and Samar and Dre again so that they can close both time loops. However, finding the right algorithm to get the reactor running again involves Tree having to die multiple times until she can memorize the right information that will give them the algorithm. Tree then goes through a suicide montage, with deaths such as electrocuting herself in the tub, drinking drain cleaner, diving into a wood chipper, skydiving in her underwear with no parachute (while flipping the bird and deliberately landing in front of Carter and Danielle), and also jumping to her death. However, when she wakes up again, the added trauma from her constant dying continues to take its toll on her, and she collapses in Carter's arms before he takes her to the hospital.
Tree is met by Gregory (Charles Aitken), who doesn't know who tree is since he is not her student, and it is also Lori with whom he is having an affair. Tree tells him to go get Lori away from Tombs, as she is escorting him to the operating room. Tree runs out and knocks the cop out to take his gun. When she gets to the OR, Lori is already dead. Tombs appears behind Tree, but she shoots him twice. She asks who freed him, right before Babyface appears and tries to grab the gun out of Tree's hand. She sees a tank of flammable gas and shoots it to kill them both.
Tree wakes up and pleads with her friends to fix the loops fast, as she doesn't know how much more time she has before she stays dead. Carter follows her on the way out, and she tells him that they are together in the other time line, and that's why it's hard for her to make a choice between him and her mom. Carter tries to tell Tree that she is living someone else's life and that she should not stay in the past. Tree leaves goes to meet with her parents for lunch and starts to feel pain again. Tree asks her parents to take her away from campus. As they drive, Julie mentions eating a treat with Tree for their birthdays, but Tree realizes what Carter meant, as these memories are not her own. The family stops at a motel to rest. Tree gets a text from Ryan saying they are close to closing the loops. While David is in the bathroom, Tree talks to Julie and asks her what she would do if she never met David. Julie says she cannot imagine a life without him, and tells Tree how she changed her life after she was born. The tender moment is interrupted when Tree sees on TV that Tombs has escaped and has killed Lori, the cop, and Carter, who went to stop Tombs. Tree realizes she has to stop the loop from closing, or else Carter and Lori will stay dead. She grabs the car keys and rushes out, trying to reach Ryan but failing as he and the others are gearing up for the reactor to go off. Tree then drives toward an electrical plant and charges into a box just a second before the reactor goes off.
Tree is revived and goes to tell Ryan the algorithm, but this time, she has decided to return to her original time line. Bronson and two security guards then come in to seize the reactor a whole day earlier than planned. Needing an urgent plan to steal it back, Tree gets Danielle to pose as a blind French student to distract Bronson while they take his keys and get into the office where the reactor is before taking it back to the lab.
Before returning home, Tree goes around trying to get closure. She approaches Lori and sincerely talks to her about ending her affair with Gregory, as she knows it's not worth it. Tree then gives Lori a hug. She then goes to Danielle's room to say how wrong she was about her, only to find a guy emerging from the bathroom with nothing but a towel, and Tree sees that Danielle sucks in both dimensions. Finally, Tree goes to have the birthday lunch with her parents, but she asks David to let her have a moment alone with Julie, which is basically Tree having one more final moment with her mother.
Tree goes to the hospital that night to save Lori. She gets to the room with the cop's gun and kills Tombs before he can attack. The ladies run out but are cornered by Babyface. Tree correctly guesses that the killer here is Gregory, wanting to silence Lori for the affair. Before they can run, Gregory's wife Stephanie (Laura Clifton) shows up to shoot Lori. She hands the gun to Gregory to kill Tree, but he shoots Stephanie first. Tree runs and grabs a weapon, and Gregory goes after her. He gets Tree in a room containing a high-powered magnet that he doesn't notice...but Tree does. She holds a screwdriver as he draws his gun on her, but she has some final words for him: "You're screwed." With that, she turns the magnet on, and Gregory gets caught in it by a metal chair. Tree releases the screwdriver, which runs through Gregory's chest, killing him.
Carter shows up, and he and Tree help Lori onto a stretcher. Tree tells Lori she is sorry for how things turned out, and how she wishes things could be different, even though Lori doesn't know what she means. Meanwhile, Ryan, Samar, and Dre have gotten the reactor up and running, but Bronson and the guards are trying to stop them again. Samar and Dre hold them off as long as they can while the reactor turns on. Tree tells Carter again that they are together in the other time line, and they share a kiss right just as the reactor reaches its mark, a mere second before Bronson can pull the plug.
Everyone wakes up in the room right before the first blast. Tree looks over to Carter and asks, "Danielle?" He responds with, "Who?" and she gleefully pounces on him as she sees she is back home.
During the closing credits, Tree, Carter, Ryan, Samar, and Dre are forced to do community service for turning on the reactor. A couple of vans pull up, and an agent from DARPA comes to escort the five to headquarters. They have taken the reactor and commend Ryan for his scientific breakthrough. DARPA wants to conduct an experiment with it, meaning they need a human subject to trap in a time loop. Tree says she has the perfect candidate.
The final shot shows Danielle waking up in bed screaming.