The homeworld of the Maximals, an advanced race of Cybertronians with beast modes, comes under attack from the planet-eating dark god Unicron. His heralds, the Terrorcons, and an army of Predacon scorpions, led by Scourge, seek to obtain for their master the Maximals’ greatest piece of technology, the Transwarp Key, which can open portals through space and time. The Maximal leader Apelinq sacrifices himself to allow the other Maximals to escape the planet before Unicron devours it. Now under the command of Optimus Primal, the Maximals use the key to flee to Earth.

In 1994 Brooklyn, ex-military electronics expert Noah Diaz struggles to find a job to support his family and is convinced by his friend Reek to steal a Porsche 911 to sell, only to discover that the car is the Autobot Mirage in disguise. Concurrently, museum intern Elena Wallace studies an ancient statue of a falcon bearing the Maximal symbol and accidentally breaks it open to reveal half of the Transwarp Key hidden inside. The key releases an energy pulse that is detected by Optimus Prime, who summons the other Autobots; Mirage is contacted in the middle of Noah’s attempted theft, and Noah is roped into the robots’ mission to recover the key so they can use it to return to their homeworld, Cybertron.

Drawn by the key’s signature, the Terrorcons arrive on Earth, and Elena is caught up in the conflict when the villains attack the Autobots outside the museum. Scourge deactivates Bumblebee and steals the key half before the Maximal Airazor arrives and drives him off. Airazor explains to the Autobots that the Maximals have been hiding on Earth for thousands of years, and split the Transwarp Key in two to keep it out of Unicron’s hands. Despite the danger, Optimus Prime is insistent that the key be reassembled so the Autobots can use it to return home, while Noah secretly plots to destroy the key to keep Earth safe.

Elena’s studies allow her to deduce that the other half of the key must be located in a hidden temple in Peru, and the group heads out aboard the rattletrap cargo plane Autobot, Stratosphere. In Peru, they meet up with locally-stationed Autobot Wheeljack, who leads them to the temple, but it is soon discovered that the other half of the key is no longer there. The Terrorcons attack once again, and the battle ends with Scourge corrupting Airazor with Unicron’s dark energy. Subsequently, the Autobots meet Optimus Primal and the other Maximals, who explain that they moved the second half of the key, entrusting it to a human tribe they have worked with over the millennia. Scourge’s corruption overtakes Airazor, driving her mad, and Primal is forced to kill her to save Elena. In the chaos, Noah attempts to destroy the second half of the key but is convinced not to by Optimus. It is instead stolen by Scourge, who then reassembles the two halves atop a volcano, erecting a tower and opening a portal above Earth through which Unicron will soon arrive.

Seeing where their self-serving motives have led them, Optimus Prime and Noah agree to work together to defeat the forces of evil. While the Autobots and Maximals battle the Terrorcons and Predacons, Noah and Elena sneak in close to the Transwarp Key, planning to deactivate it with an access code Elena has uncovered. In the battle, Mirage is gravely wounded by Scourge but transforms his damaged body into a powered exo-suit for Noah so they can fight together. The energy pulse released by the key activates a dormant Energon deposit beneath the valley where Bumblebee was kept, which infuses his deactivated body with Energon and restores him to life, his revival turning the tide of the battle. Optimus Prime kills Scourge, but not before the latter damages the control console to prevent it from being shut down. Willing to sacrifice himself, Optimus destroys the key and collapses the portal, but Noah and Primal are able to save him from being sucked into the imploding vortex.

In the aftermath of the conflict, the Autobots, now without means to return to Cybertron, proclaim Earth as their new home and vow to continue protecting it along with the Maximals. Elena receives recognition for discovering the temple in Peru while Noah attends an interview for a security job but finds he is actually being invited to join the secret government organization G.I. Joe.

In a mid-credits scene, Noah manages to repair Mirage using junk Porsche parts from Reek, who learns that the car is a Transformer.