In 484 AD, the legendary King Arthur (Liam Garrigan) teams up with the wise wizard Merlin (Stanley Tucci), who forges an alliance with the Knights of Iacon—a formidable group of twelve Transformers hidden on Earth. The Knights bestow upon Merlin an extraordinary alien staff and transform into a colossal dragon, aiding Arthur in his battle against the Saxons.
Fast forward to the present day, where governments worldwide have branded Transformers as outlaws, leading to the formation of the Transformer Reaction Force (TRF), dedicated to exterminating these alien beings. Although Optimus Prime has departed Earth searching for his creator, new Transformers continue to touch down on the planet. The latest craft, however, crashes in the wreckage of Chicago, where a group of children stumbles upon it. The kids find themselves in peril as a TRF Mecha confronts them, but they're rescued by Izabella (Isabela Moner), a brave survivor of the Battle of Chicago, along with her trusty Transformers, Sqweeks and Canopy. Tragically, Canopy is slain by the TRF during their escape. Just then, Bumblebee and Cade Yeager (Mark Wahlberg) arrive to assist, although Cade cannot save the injured Transformer onboard. In a final act, Canopy attaches a metallic talisman to Cade’s body, witnessed by Barricade, a Decepticon scout who quickly informs Megatron of this critical development.
In the far reaches of the Solar System, Optimus discovers that their home world, Cybertron, is disintegrating and hurtling towards Earth. He encounters Quintessa, a sorceress who claims to be his creator. With ambitions to harness the staff’s powers that were stolen from her and given to Merlin, she puts Optimus under her control, renaming him Nemesis Prime and commanding him to retrieve the staff. To make matters worse, Quintessa reveals that Earth itself is known as Unicron, Cybertron's ancient foe, and she plans to siphon Earth’s life force to restore Cybertron.
Meanwhile, enormous horns emerge at six locations globally, with the Chinese dubbing them "The Horns from Hell." NASA deduces that these horns are spawning an inter-galactic dimension that Cybertron will enter in just three days.
William Lennox (Josh Duhamel), a TRF member and former ally of the Autobots, brokers a deal between the TRF and Megatron, releasing a cadre of Decepticons to aid in the retrieval of the talisman from Cade. Following orders, William discloses Cade's whereabouts to ensure that Megatron finds the staff, anticipating that the U.S. military will step in during the chaos.
The hunt for Cade leads Megatron to his junkyard sanctuary in South Dakota, where he and several surviving Transformers take refuge. After a tense standoff, Cogman, a Transformer emissary of British Lord Sir Edmund Burton (Anthony Hopkins), arrives to escort Cade and Bumblebee to England to meet his master. Upon arrival, Burton unveils a shocking revelation: he is the last living member of the Witwiccan order—a venerable brotherhood tasked with safeguarding the Transformers' secret history on Earth. To his astonishment, he informs Cade that Viviane Wembly (Laura Haddock) is the last descendant of Merlin and holds the key to wielding the staff to avert the impending doom posed by Cybertron. The staff, fused with Merlin's DNA, lies buried with him, safeguarded by the alien knights.
As they evade the TRF, Cade and Viviane uncover clues left by her father, which lead their team—Bumblebee, Cogman, and themselves—to venture into the depths of the sea aboard the submarine HMS Alliance. Their mission: to locate the Cybertronian Knights' sunken vessel, where they ultimately discover Merlin's tomb and his powerful staff. When Viviane activates the staff, the ancient ship resurfaces just as the TRF intervenes, leading to a confrontation.
In the heat of battle, Optimus—now under Quintessa's control—confronts Bumblebee. At a crucial moment, Bumblebee speaks, breaking the enchantment and shattering Quintessa's hold over Optimus. Megatron, revealed to be colluding with Quintessa all along, seizes the staff and escapes, but the Knights rise to challenge Optimus. In a spectacular turn of events, Cade's talisman transforms into Excalibur, and he harnesses the power of the Knights, compelling them to yield to him and urging Optimus to once again protect Earth.
As Megatron delivers the staff to Quintessa, intending to drain Unicron's life force via Stonehenge, the British military steps in. However, Megatron fatally shoots Burton, who perishes with Cogman at his side. The Autobots arrive with reinforcements, engaging in an epic showdown against the Decepticons and Quintessa's minions. With the Knights' support, Optimus confronts Megatron, ultimately sending him crashing back down to Earth while Bumblebee seemingly takes down Quintessa.
With Viviane extracting the staff, the imminent destruction of Earth is halted, albeit at the cost of Cybertron's destruction. In a powerful message of unity, Optimus proclaims that humans and Transformers must collaborate to rebuild their worlds and sends out a call to any surviving Autobots to return home.
In a riveting mid-credits scene, scientists investigate one of Unicron’s protruding horns from the desert. Meanwhile, Quintessa, having survived and now disguised as a human, offers them a method to eliminate Unicron.
