Earth, 65 million years ago: the Power Rangers of the era perish one by one at the hands of their ally turned traitor, the Green Ranger, Rita Repulsa. Zordon, the Red Ranger, is the last one left to oppose Rita, but he is gravely wounded. With the last of his strength, Zordon hides his Power Coin, along with those of his allies before calling upon his last living comrade, the android assistant Alpha 5, to call down a meteor strike. The very same meteor that drove the dinosaurs to extinction killed Zordon and sent Rita plummeting into the bottom of the ocean, never to threaten the Earth again...for a time.
Angel Grove, America, the present day: a high school football star named Jason Scott is thrown off of his team, placed under house arrest, and ordered to attend detention on Saturday after a failed prank involving a bull, to the chagrin of his father. At detention, he makes the acquaintances of Billy Cranston, an autistic youth who is highly intelligent and serving detention after an incident where he blew up a locker. After Jason drives off a bully who harasses him, Billy offers to disable his ankle monitor so that he can help him with a project he had been working on.
The two travel to an abandoned gold mine, where Jason encounters another student from detention: Kimberly Hart, a former cheerleader. When Billy sets off a homemade remote charge, he accidentally attracts the attention of two other students who are truant from school: Zack Taylor and Trini. The explosion uncovered five colored medallions, which they take before making their escape after the explosion alerts security. The five attempt to make their escape in a van Billy "borrowed" from his mother, but the van is hit by a train.
Jason and the others awaken in their bedrooms. Not only are they unscathed in spite of being in a train collision, but each one now possesses superhuman strength.
Meanwhile, out at sea, Jason's father, who works as a fisherman, uncovers a cadaver in his fishing haul. He keeps the body in the ship's hold to have the police look at after returning to land, unaware of the fact that the supposed corpse is actually the body of Rita Repulsa, trapped in hibernation for the last several million years, and now beginning to reawaken...
Jason, Billy, and Kimberly agree to return to the mine after detention to figure out the cause behind their survival and their superhuman strength, which seem to be tied to the medallions they unearthed. There, they also find a similarly curious Zack and Trini. They dive into a cave, where they find an ancient spaceship. Contained within is Alpha, who awakens Zordon's consciousness, which was stored in the ship after his death. Zordon tells the teenagers of the Power Rangers, the intergalactic protectors of peace throughout the universe, and warns them of Rita, who plans to gather gold in order to awaken an ancient monster known as "Goldar". With Goldar awakened, she would seek out the Zeo Crystal, which contains incredible power and, if uprooted from the Earth, would destroy all life on the planet.
Billy, Zack, Trini, and Kimberly are reluctant to buy into Zordon's story and leave. Zordon beseeches Jason, whom he recognizes as a natural born leader, to convince them to stay. Although it is a hard sell, Jason manages to convince the others to return to the ship the following day, so they could prepare to fight Rita.
In order to fight Rita, Jason and company would need to learn how to "Morph"; to summon suits of armor that would allow them to defeat Rita. In order to prepare them for the battle and try to bring out their ability to Morph, Zordon and Alpha have the five prospective Rangers fight against simulated versions of "Putties", Rita's earthen foot soldiers. Over the next several days, the teenagers train, but are unable to Morph. To try to inspire them, Zordon and Alpha show the five the giant biomechanical battle machines used by the Power Rangers: the Zords.
Zack is immediately taken with the Zords and attempts to take one for a joyride, but ends up being unable to control it and nearly kills everyone trying to pilot it. He and an enraged Jason get into a fight, which Billy desperately tries to break up. In doing so, Billy unwittingly Morphs, donning blue armor, but only for a few brief moments, and he cannot replicate the feat. An exasperated Zordon instructs the five to return home afterwards.
Before Jason leaves, however, he overhears a conversation between Zordon and Alpha revealing the real reason they were training: if Jason and his friends could Morph, it would open the Morphing Grid and give Zordon the opportunity to return to life. Jason calls Zordon out on his self-centered reasoning for making them fight Rita and storms off.
That night, the group theorize that they can't Morph because they aren't open with each other. To that end, they arrange to have a campfire where they reveal their secrets to one another. Zack confides in the group that he is the son of a single mother who is seriously ill, and he spends a lot of time away from home out of fear of his mother's passing away while he was under the same roof. Billy admits to liking country music, a taste he acquired from his late father. Trini tells the others that her parents are very strict and possibly disapprove of her sexuality. Kimberly hesitates at admitting her secrets, but would later admit to Jason that she had been sent to detention after an incident where she unwittingly spread nude photos of another classmate among the student body, an incident she felt immensely guilty over..
The five return home that night. Rita, who had regained her power over the course of the last several days, ambushes Trini in her room. She offers to spare Trini if she brings the other Rangers to the docks. Trini calls the other Rangers over to explain the situation, but all five end up captured by Rita, who threatens to kill them unless they tell her the location of the Zeo Crystal. She manages to get the information out of Billy, who, through his own research, determined that the Zeo Crystal was under a local Krispy Kreme restaurant. For his trouble, Rita sends Billy plummeting into the ocean. The other four Rangers wrest themselves free and pull Billy up, but it's too late: Billy is dead.
Jason, Trini, Zack, and Kimberly take Billy's body back to the spaceship and beg Zordon to revive him, but Zordon regretfully tells them that it's beyond his power. Crestfallen at the loss of a valuable teammate and a dear friend, Jason and the others admit that they would give their lives to take his place. Their love for their friend opens the Morphing Grid, and for a moment, Zordon's consciousness goes quiet. Zordon, however, doesn't use the opportunity to return to life. Instead, he uses his one and only chance at resurrection to revive Billy, to the relief of his friends.
Now able to tap into the power of the Morphing Grid, Jason and the others are ready to take on Rita: it's Morphin' time!
Rita finally resurrects Goldar, and the two make their way towards Krispy Kreme as the Power Rangers fight their way through a swarm of her Putties. As their numbers begin to seem insurmountable, Zack falls back and returns with his Zord, finishing off the Putties. The other Rangers board their Zords and make their way back to Angel Grove to fight Goldar. Rita manages to unearth the Zeo Crystal beneath Krispy Kreme, and orders Goldar to push the Zords into the crater she had created before extracting the crystal. Just as it seems like the Rangers are doomed, the Zords combine with one another to form a gargantuan humanoid battle machine that can match Goldar: the Megazord. The Rangers work together to operate the Megazord and manage to destroy Goldar. Jason gives Rita a chance to surrender, but Rita refuses, warning the Rangers that others would come for Earth's Zeo Crystal. She attempts to attack the Megazord, but is sent flying into space, towards the moon, where she would never to be heard from again.
With Goldar destroyed, Rita defeated, and the earth saved, the people of Angel Grove hail the Power Rangers as heroes. Jason returns his sword to Zordon and promises to return for it when the need arises once more, as he and his new friends return to a life of relative normalcy.
Shortly after Rita's defeat, another detention roll call seeks out another student: Tommy Oliver...