The film opens in the Cenozoic Era on Earth. A great battle has just taken place. The Red Ranger, Zordon (Bryan Cranston), crawls over to a dying Yellow Ranger crawls to him and de-morphs. Before she dies, Zordon apologizes for his failure to save them. After she dies, he communicates with Alpha 5 to send a meteor to destroy all life on Earth, and demorphs as he sends his and the rest of the Ranger's power coins away, instructing them to seek those who are worthy of them. The Green Ranger, Rita Repulsa (Elizabeth Banks), appears to him, stating that with the Zeo Crystal, he and her can rule the Universe and the World. He tells her that will never happen. After telling him that he will die, he responds that they will die together. A huge blast then goes off, sending Rita into the ocean along with her power coin.
It's the present day in a small town called Angel Grove. Star football player Jason Scott (Dacre Montgomery) and his buddy are sneaking a cow into the locker room of another school as a prank. As they hear sirens, they escape the school as they evade the police (with the cow leaving). Jason drives away recklessly and ends up getting into a pretty bad car accident as his car flips over.
Three weeks later, Jason is forced to attend weekend detention classes for the rest of the school year, and he is also placed under house arrest. In the car, his father Sam (David Denman) scolds Jason for always making stupid choices his life, each one being more stupid than the last one. Jason gets wise. Sam warns him not to. He also knows that, despite Jason saying other wise, that he didn't act alone on it. He goes on how he completely threw away his prospects and that he has to spend time with other degenerates. He believes that he and Jason will never understand each other, with Jason agreeing. In detention, he sees Billy Cranston (RJ Cyler) organizing his pencils by color. A bully (Wesley MacInnes) comes over and starts harassing Billy by breaking one of his pencils and calling him a freak. Jason steps in and defends Billy. The bully tries to swing at Jason but he gets smacked hard across the face before being told to not bother either of them again.
Kimberly Hart (Naomi Scott) leaves the room before their session begins. She goes to the ladies room where she meets up with two of her old friends, Amanda (Sarah Grey) and Harper (Morgan Taylor Campbell). They have decided to kick Kimberly off the cheerleading squad, and out of their own lives completely, due to Kimberly distastefully sharing a sensitive photo of Amanda to Kimberly's boyfriend, who in turn, spread it around to the entire school. Harper spitefully cuts Kimberly out of a picture of them before leaving. Kimberly takes the scissors and starts to cut her own hair.
After the session is done, Billy catches up with Jason and thanks him for defending him. He invites Jason to hang out with him, but Jason shows him the monitor on his ankle. Since he doesn't understand Jason's metaphors and clever wordings to describe his situation, Billy reveals that he doesn't really intend for them to hang out; he needs to get somewhere that night and could use Jason's help. Jason then clarifies his house arrest to him, saying there's nothing he can do about it. Knowing how to do it, Billy offers to disable the SIM on the monitor if he goes to his house before the time the tag goes off. He also offers to lend Jason his mom's van for a few hours if he agreed to help him that night. Jason instead goes home, but after hearing his parents arguing about him, he rides his bike to Billy's house. With four minutes left, Billy calmly and carefully scrambles the SIM chip. Jason thanks Billy, and owes up to his end of the deal.
The guys go by a hill where they carry a big crate of equipment that Billy wants to use in a specific location. Jason isn't comfortable with the task, as he and Billy don't know each other. Billy explains that he's on diagnosed with Spectrum. Jason asks if it's a workout program as a joke, but Billy further explains that his brain doesn't work the same as his, leaving him unable to understand humor or sarcasm, so he didn't get the joke. After he drops off Billy, Jason leaves him there. He runs into Kimberly, who is swimming in the lake. Meanwhile, another Angel Grove student, Zack Taylor (Ludi Lin), is watching the new girl, Trini (Becky G), doing yoga nearby.
Jason and Kimberly talk about how much they hate Angel Grove for outcasting them just for making stupid mistakes and their wish of running away and starting new lives. Meanwhile, Billy works on digging through the wall of a hill with an explosive he built into the wall, and causes an explosion. Everyone runs to the scene. Zack and Trini meet the other three kids. Zack and Trini explain that they can't be out there as the place is restricted. Suddenly, the wall further collapses, revealing a crystalline wall behind it. On the wall are what appear to be different colored gems. Zack breaks them off. They are actually power coins with the colors red, blue, black, yellow, and pink. They each pick one up and keep them. When they hear alarms going off, the kids run for it. Jason and Billy take the van before going back to get the other three. Their path is headed in the same direction as an oncoming train. Billy thinks he can make it before the train comes, but the train slams right into the van. The van crashes, and the coins start glowing.
Somewhere over the ocean, Sam, who works as a fisherman, has his men pull up a big pile of fish as a storm starts hitting. He discovers Rita's corpse among the fish.
The kids all wake up in their rooms, miraculously unharmed and with enhanced strength (Jason breaks off a piece of his sink, Billy tears off the door of his tool closet, and Kimberly crushes her phone in her hand). They still have their power coins on them. At school, the bully from detention goes up to harass Billy again by trying to break his wrist, but Billy's bones don't budge. The bully then headbutts Billy, only to get knocked unconscious and have all the kids think Billy did it on his own, gaining brief popularity amongst them. Later at lunch, as he appeals to a group of kids about his amazing action, he is pulled aside by Kimberly to talk to her and Jason about their changes. When the power coins cause the lunch table to overheat, causing all the trays and food to melt or explode, they decide that they should go back to the hill to find answers.
The three return to the hill and find Zack trying to break through the wall himself. They all see Trini, but she climbs up a wall to avoid the kids. They chase after her, also climbing up the hill. Kimberly tries to talk reason with her, but Trini then makes a huge leap across a crevice. The others jump after her. Billy is reluctant and afraid at first, but after the others persuade him, he jumps, but almost doesn't make it. He pulls himself up the ledge and celebrates, only to fall down the crevice. Jason calls out repeatedly for Billy, but there's no response at first, causing them to think he's dead, but then they are relieved when they hear him telling the gang to join him. Jason and Zack jump in, but Trini starts to walk away. Kimberly tricks her into coming back so she can pull her down with her.
The kids all land in a body of water, and all start glowing colors underneath. They swim down to find an underground cave. They come across a ship with advanced technology in it. A small robot named Alpha-5 (Bill Hader), who's been waiting for "them" for the past 65 million years, finds the gang and brings them to Zordon, who now lives within the Morphing Grid to preserve his consciousness. Zordon and Alpha-5 explain to the gang that because the power coins came into their possession, they have been chosen to become the new Power Rangers. Billy is excited, but the rest of the group scoff this. To prove his point, Zordon visually introduces them to Rita and what she intends to do with the world. The group is horrified. He further tells them about Rita and how she was originally part of their team until she betrayed her fellow Rangers for control of the Zeo Crystal. She also wants to create Goldar, a gigantic golden monster that will tear apart Angel Grove to find the Crystal. Not wanting any part of this, the group leave the ship. However, Zordon stops Jason and convinces Jason to believe him. Jason then returns, surprised that the group have waited for him. He persuades them that everything they've learned in there might be true given the unexplainable changes they went through earlier that day, and, though he cannot force them to, he hopes to meet him the next day when he plans to return.
Later that day, it is revealed that Zack's mother is dying as he brings her meds to her as she lies in bed. She asks Zack if he's still going to school, but Zack doesn't answer, only telling her to take her meds. In Trini's home, her parents try to get her to talk to them about her day; her mother grows immediately frustrated with her silence, but her more patient father tries to calm down the situation and tries again a little more calmer. She tells them exactly what happened. Her younger twin brothers are amazed, but her mother doesn't believe her, and orders her to urinate in a test cup to make sure she's not on drugs. Sam has a cop come onto his boat to check on the corpse they found. Tired from the long night, Sam goes home to rest and leaves the boat to the cop alone. While investigating, Rita's coin revives her, and she attacks and kills the cop.
Jason goes back, and is satisfied to see that the others decided to return with him. They go into the ship, where Zordon instructs them to step into the morphing grid to morph into their armor so they can officially become the rangers, but the morphing grid fails. Zordon then instructs them to go into The Pit, where they are forced to fight against simulations of Rita's rock monster minions, the Putties. The Rangers get beat up but they continue training and try to morph, but with no luck.
Rita has been devouring gold off of several victims to gather for Goldar. She makes it into town and enters a jewelry store, where she starts consuming more gold and adding it to her staff. An officer arrives and orders Rita to stand down. She creates a Puttie and has it kill the officer.
In an attempt to inspire them, Alpha-5 shows them their Zords, which are huge mechas that the Rangers can control, but Alpha-5 and Zordon feel they aren't ready to use them. Zack doesn't listen and decides to take his Zord out for a joy ride. He is unable to control it and ends up running around outside the cave, nearly running into town before crashing back into the cave, nearly crashing into everyone. Jason gets angry with Zack's carelessness almost causing their lives, and the two start a fight. When Billy intervenes, the others see that he has morphed, but only temporarily. Disappointed with their lack of progress, Zordon instructs the group to leave. Jason tries to get Billy to morph again to prove their progress, but Billy is unable to. Zordon believes that they are not Rangers.
As they leave, Jason goes back to the ship where he catches Zordon and Alpha-5 talking about that he can't be physically revived without the morphing grid working. Believing that they aren't destined to be Rangers and Zordon made it up for his own personal agenda, he angrily walks out on him, being done with Zordon. Zordon tries to warn Jason about Rita's threat and that the team needs to be ready for her, Jason refuses to listen.
Zack sets up a campfire outside the cave and invites the gang to hang with him. They decide to open up and get to know each other because they feel that's why they're not morphing. Zack starts and discusses his ailing mother while expressing his fears over what will happen to him if something happens to her. Billy then opens up about him not missing his deceased father, as other than going to the mines with him, they never really had much of a relationship outside of that. When Jason asks, Billy reveals that the reason he was in detention was because he blew up his lunch box resulting from a failed experiment. Kimberly skips her turn, so Trini talks about how she's moved around a lot regarding personal issues. She reveals that she's secretly question her sexual orientation, and is too afraid to open up to her stereotypically normal parents, who are already having a hard time accepting her without knowing. Jason acknowledges how people had seen him and how people may see him now as a result of his mistakes.
Rita attacks Trini in her bedroom. She taunts her with the fact that she killed the last Yellow Ranger. Trini and Rita fight for a bit, but Rita overpowers Trini. She asks Trini where her crystal is. Trini doesn't know, and Rita believes her. She demands her to follow her orders.
Kimberly goes to Jason's house and admits to him why she was really in detention. The main reason was because she punched her boyfriend's tooth out, but that was because he called Kimberly the meanest person ever. The reason for THAT was because Amanda, Kimberly's previously seen friend, sent her an explicit picture in confidence, only for Kimberly to spread that picture to her boyfriend, who would also spread that picture around to everyone else at school. When confronted about it in the office, she was faced with the guilt of being there when the photo was shown to Amanda and her father. She thinks it's her fault that they aren't morphing. Jason tells her to move on from that, as this one foolish action doesn't define her as a person.
Trini tells everybody to meet them at the docks. She tells everybody about her incident with Rita, and Jason reveals what he learned from Zordon. He then decides to stop Rita anyway with various tools lying around, only for Rita to amush them all. She knows that Billy knows where the Crystal is. He discovered earlier that the Crystal lies beneath none other than...a Krispy Kreme. Rita heads off to find the Crystal but sends Billy underwater with some of her deadly energy from her staff. The others free themselves from their binds and try to pull Billy out. They try to revive him, but he is already dead.
The four carry Billy's body to the cave. Jason tries to get Zordon to revive him, but without the morphing grid working, Zordon is unable to. Jason expresses his sorrow and feelings that he failed Billy and his friends. The rest of the group lament, saying that it all should've been them instead of Billy, since Billy was the most innocent of them. The Morphing Grid then activates, giving Zordon an opportunity to come back with his own body. After a brief moment, Zordon's face disappears from the wall. It then returns, and Alpha-5 asks Zordon why he didn't come back. He says only one may return. Billy then wakes up, to the delight of his friends. Now that he sees they can work together as a team, Zordon tells the Rangers to step onto their pads. They finally morph into their suits and head off to stop Rita.
The Putties attack the Rangers outside the cave as Rita is in the process of creating Goldar. The rock monsters outnumber the Rangers, so they hop in their Zords and beat them before heading into town. Goldar is already destroying Angel Grove to find the Crystal. The Rangers head in and fight Goldar separately. Jason spots his father caught up in the chaos and he goes to rescue him, not revealing who he is. Goldar takes down Kimberly's jet and knocks the other Zords into each other. Goldar then starts to push the Rangers into a fiery pit as they start to accept their fates. Goldar knocks them all in, but they suddenly join their Zords together to create the Mega-Zord.
Rita finds the Krispy Kreme and has Goldar tear it down to get the Crystal. However, the Rangers use the Mega-Zord to fight Goldar. Rita then lets Goldar absorb her so that she may give the creature more power. The Rangers overpower Goldar and use their giant swords to destroy the monster. The gold melts off Rita and she faces the Rangers. Jason orders her to surrender so that they may take her to be judged by Zordon. Rita refuses and she leaps to attack the Rangers. The Mega-Zord simply smacks Rita into outer space where she is frozen. A crowd of citizens (including the original Green and Pink Rangers - Jason David Frank and Amy Jo Johnson) gather in awe of the Rangers.
The town recognizes the Rangers as their mysterious saviors. Sam hangs an article of their heroics on his fridge, implying he knows Jason is one of them. As the Rangers continue their regular lives, Trini is forced to repair the walls of her bedroom as a punishment, Zordon's voice narrates over as he expresses gratitude to the five friends for carrying on the legacy of the Power Rangers.
In a mid-credits scene, the teacher in detention is calling for Tommy Oliver, but he is not there. Moments later, Billy's locker blows up as a result of another one of his experiments.