The homeworld of the Maximals, an advanced race of beast-mode Cybertronians who have evolved on a jungle planet colonized by Transformers long ago, comes under attack by the planet-eating dark god Unicron. His heralds of him, the Terrorcons and an army of Predacon drones, 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 the Unicron devours him. Now under the command of Optimus Primal, the Maximals use the key to flee to Earth.

In Brooklyn in 1994, ex-military electronics expert Noah Diaz struggles to find a job to support his family, and his friend convinces him that it sucks to steal a Porsche to sell, only to discover that the car is the Autobot Mirage in disguise. At the same time, museum intern Elena Wallace studies an ancient statue of a falcon bearing the Maximal symbol and ends up breaking it to reveal the half of the Transwarp key hidden inside. The key releases a pulse of energy that is detected by Optimus Prime, who summons the other Autobots; He contacts Mirage in the midst of Noah’s attempted robbery, and Noah ends up being tied into the robots’ mission to retrieve the key so they can use it to return to his homeworld of Cybertron.

Lured by the key’s signature, the Terrorcons arrive on Earth and Elena is drawn into the conflict when villains attack the Autobots outside the museum. Scourge kills Bumblebee and steals half of the key before Maximal Airazor arrives and chases him away. 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 reach. Despite the danger, Optimus Prime insists that he reassemble the key so the Autobots can use it to return home, while Noah secretly plans to destroy the key to keep the world safe from him.

Elena’s studies of her lead her to deduce that the other half of the key must be located in a hidden temple in Peru, and the group heads aboard the dilapidated Autobot cargo plane, Stratosphere. In Peru, they meet a locally stationed Autobot Wheeljack, who takes them to the temple, but it is soon discovered that the other half of the key is no longer there. The Terrorcons attack once more and the battle ends with Scourge corrupting Airazor with Unicron’s dark energy. Afterwards, the Autobots meet Optimus Primal and the other Maximals, who explain that they moved the second half of the key and entrusted it to a human tribe they have worked with for millennia. Scourge’s corruption reaches Airazor, driving her insane, and Primal is forced to unplug her to save Elena. In the chaos, Noah attempts to destroy the second half of the key, only to have it stolen by Scourge, who then reassembles the two halves on top of a volcano, opening a portal above Earth through which Unicron will soon arrive.

Seeing where their selfish motives have led them, Optimus Prime and Noah agree to work together to defeat the forces of evil. As the Autobots and Maximals battle the Terrorcons and Predacons, Noah and Elena sneak up on the Transwarp key, planning to disable it with an access code that Elena has discovered. In the battle, Scourge severely injures Mirage, but transforms her 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 reservoir below the valley, infusing Bumblebee’s body with energy and bringing him back to life, his rebirth turning the tide of battle. Optimus Prime kills Scourge, but their fight ends up destroying the key’s control console, meaning it cannot be turned off. Willing to sacrifice himself, Prime destroys the key and collapses the portal, but Noah and Primal are able to save him from being sucked into the implosion vortex.