The film starts on Cybertron, in Iacon City. A miner bot named Orion Pax (Chris Hemsworth) sneaks into an old building. In it is an archival recording that details the planet’s origin. The first Transformer, Primus, used his body to create Cybertron, and from the planet’s core came more Transformers, including the 13 Primes. The Matrix of Leadership was forged as a major source of power, but after it was lost in time, the planet’s source of Energon stopped flowing. Two larger Transformer guards catch Orion sneaking around, forcing him to run.
Orion runs through the city until he runs into his best friend, fellow miner D-16 (Brian Tyree Henry), who causes a distraction to allow Orion to sneak into a cart, evading the guards. D-16 idolizes Sentinel Prime (Jon Hamm), the leader of Cybertron, though his true hero is Megatronus Prime.
The two bots head to work, where they and other bots are made to mine Energon. The group is led by the no-nonsense Elita-1 (Scarlett Johansson). A part in the mine gets hit, which triggers an unstable reaction. The bots start to head out of there, but Orion and D-16 stop to help Jazz (Evan Michael Lee), whose leg gets trapped under rubble, even though Elita told them to not stop for anything. The two manage to save Jazz and get themselves out of the mine before it caves in. A high-ranking Transformer named Darkwing (Isaac C. Singleton, Jr) fires Elita. Orion talks back to Darkwing and almost gets punched, but D-16 punches Orion for Darkwing…and Darkwing returns the favor by punching D-16. He is annoyed, but Orion later thanks him for having his back.
Sentinel Prime broadcasts a message to Iacon City as he is searching for the Matrix of Leadership to revive the flow of Energon. He also reminds everyone of the upcoming Iacon 5000 race for the next day. At night, Orion proposes to D-16 that they enter the race, but D-16 thinks that they are better off where they are at.
The next day, Orion and D-16 head to watch the Iacon 5000, but Orion surprises D-16 by showing off that he got them jet packs so they can enter the race. D-16 reluctantly joins him, and the two quickly get the attention of everyone watching, including Elita and Sentinel, as well as their fellow miners. After losing their jet packs, the two improvise by boosting themselves back into the race using a thruster, and then taking advantage of the other Transformers getting pulled in by magnets. Orion and D-16 run towards the finish line, only to get beaten by Darkwing…as well as everyone else.
D-16 is mad at Orion for talking him into the race, fearing that they will get in so much trouble. Instead, Sentinel shows up to commend the two for putting on a show. He invites them to join him, only for Darkwing to throw them to a lower level, where they are demoted to smelting. There, they meet B-127/”Bee” (Keegan-Michael Key), a chatty and eager bot. Orion and D-16 find that Bee has a hidden distress beacon from Alpha Trion (Laurence Fishburne), one of the Primes. The beacon produces coordinates to the surface, where Orion believes that they will find the remains of the Primes, as well as the Matrix of Leadership. He convinces a reluctant D-16 to join, while Bee is more than ready to get out of there.
The three bots sneak past security as they board a train to the surface, but they are spotted by Elita, who follows them. While she is prepared to confront them, she joins them in awe as they see the world above the surface for the first time. A surge of Energon causes the landscape to grow at a rapid pace, causing the bots’ train to crash.
The four start making their way forward, hiding from a ship belonging to the Quintessons, the longtime enemies of the Transformers. After evading capture, the bots come upon a cave containing the corpses of the Primes. Orion uses a small sample of Energon to revive Alpha Trion. He awakens distraught to discover his brothers long dead, and the Matrix long gone. When Trion learns that the four work under Sentinel, he is quick to refer to Sentinel as a false Prime and a traitor. Using a recorded memory, Trion shows the four that the Primes battled the Quintessons in the war for Cybertron, only for Sentinel to betray them and align himself with the enemy to live like a king. After he killed the other Primes, Sentinel tried to take the Matrix for himself, but since the Matrix chooses who can use it, it deemed him unworthy and vanished from existence.
After hearing noise, the four go outside the cave to find Quintesson ships descending to meet with Sentinel himself. He presents them with the harvested Energon, but the Quintesson leader nearly kills Sentinel for the low supply. D-16 becomes enraged to learn that Sentinel is responsible for the drought of Energon, and for the fact that Sentinel took the transformation cogs from each of the bots to prevent them from taking on their more powerful forms. Trion gives Orion, D-16, Bee, and Elita the cogs of the other Primes, officially making them Transformers. As they take their new forms, Sentinel’s goons, led by Airachnid (Vanessa Liguori), go after them. Trion holds them back in his beast form to fight them before Airachnid overpowers him. The four bots try to get the hang of their new powers while evading more goons. After getting to safety, D-16 vows to make Sentinel pay for his crimes by killing him.
Airachnid brings Trion before Sentinel, who taunts the fallen Prime over the deaths of his brothers. Trion calls Sentinel a traitor once again before he executes Trion. He then sends Airachnid to find the bots.
Orion tries to talk D-16 down from his anger. With the recorded memory on hand, he hopes that they can show proof of Sentinel’s treachery to Iacon City, but D-16 refuses to follow any leader ever again. Moments later, they, plus Bee and Elita get captured. They awaken to find themselves with the Cybertronian High Guard, led by Starscream (Steve Buscemi) and his top guards Shockwave (Jason Konopisos-Alvarez) and Soundwave (Jon Bailey). They have been in hiding from Sentinel, for which D-16 calls them out on. Starscream takes his challenge and fights D-16, only to get overpowered and have his voice box damaged. D-16 then rallies the rest of the High Guard to look to him as their new leader so that they can come out of hiding and destroy Sentinel once and for all.
Airachnid, joined by both Sentinel’s troops and Quintesson ships, locate the bots and the High Guard, launching an aerial attack. The bots go into battle, now utilizing their newfound Transformers weaponry, but D-16 and Bee are among the captured prisoners after Sentinel’s forces lay siege to the hideout. The recorded memory has also been destroyed, leaving them without their proof. Orion feels responsible for what has happened, but Elita talks him out of his feelings to inspire him to return and get help. Orion gathers Shockwave, Soundwave, and the remaining High Guard to go back to Iacon City.
The prisoners are brought before Sentinel. D-16 wastes no time in letting his former hero know what he thinks of him now. Sentinel tries to knock D-16 down, but he refuses to kneel and gets back on his feet. Sentinel then sees the image of Megatronus Prime that D-16 keeps, and he brands it onto his chest.
Orion and Elita make it to the mines (after Orion knocks Darkwing’s lights out) to round up the rest of the miners, filling them in on Sentinel’s betrayal and why they need to stop him the right way. The other bots cheer Orion on and follow him into battle.
As the bots make their way into the city, they are spotted by Airachnid. The hero bots fight Sentinel’s forces while he takes on his final winged form to battle Orion and D-16. Elita fights Airachnid until she manages to subdue her and bring her with Bee to a tower, where they broadcast her memory logs and show Sentinel admitting to his crimes. The city quickly turns on him, and in his distracted state, D-16 attacks him and prepares to strike the killing blow. Orion stands in his way to try and talk him down again, but D-16 is too far gone to back down. He raises his gun arm and fires a shot at Sentinel, but Orion takes the shot, dealing a heavy blast to his body. D-16 grabs Orion before he falls down a deep chasm, only for his eyes to glow red and declare that he is done saving him. D-16 drops Orion down the hole in front of Bee and Elita before proceeding to tear Sentinel in half and ripping out his cog, which he in turn stole from Megatronus Prime. D-16 addresses his followers by his new name…MEGATRON.
Orion falls deeper and deeper until he goes nearer to the planet’s core. A beam of light pulls him closer, and Orion is met by the spirits of the 13 Primes, as well as Primus himself. For his sacrifice, they have deemed Orion worthy of inheriting the Matrix of Leadership and being the one to bring back Cybertron. Thus, Orion Pax is reborn as…OPTIMUS PRIME!
Megatron is joined by Shockwave, Soundwave, and the other High Guard in laying siege to the city and tearing down Sentinel’s monuments. Bee and Elita make an effort to fight them off, while Optimus rises from below more powerful than ever. The former friends battle it out until Optimus uses his swords to destroy Megatron’s guns. Optimus banishes Megatron and his forces from the city, and the two part on bitter terms as sworn enemies.
Optimus uses the Matrix to revive the flow of Energon across Cybertron. He appoints Elita to be his commander, while Bee is made his right-hand bot. Optimus then redistributes the transformation cogs to all of the miners, turning them into his new team: the Autobots. While he laments the loss of his best friend, even remembering the first time they met, Optimus vows to lead the fight for Cybertron against the Quintessons and all other enemies. The Autobots then proceed to ROLL OUT.
Mid-Credits Scene:
Bee goes back to his old mining level with the fake friends he made, only to accidentally “decapitate” them with his laser-knife hands.
Post-Credits Scene:
Megatron gathers the High Guard in a revenge mission against Optimus and the Autobots. They rename themselves the Decepticons and set their plans in motion.
