| Director | Michael Bay |
|---|---|
| Budget | $217 million |
| Box office | $605.4 million |
In 484 AD, Merlin finds the Knights of Iacon, a group of Transformers hiding on Earth, seeking their help to aid King Arthur and his knights. They hand him a staff and help Arthur defeat the Saxons, but warn Merlin to hide the staff.
In the present day, following the events of Hong Kong, Optimus Prime arrives on the ruins of Cybertron and meets the Cybertronian goddess Quintessa, who brainwashes him into becoming Nemesis Prime and sends him to Earth to retrieve Merlin's staff, which will restore Cybertron by taking Earth's energy core.
On Earth, a new paramilitary task force called the Transformers Reaction Force (TRF) continues to hunt Transformers, while some of its U.S. military personnel, including former NEST members Colonel William Lennox and General Morshower, are reluctantly against its actions. Cade Yeager, an ally to the Autobots, hides Transformer refugees in his junkyard. In a war-torn Chicago, Cade and Bumblebee meet a scavenger named Izabella and her Transformer companion, Sqweeks, before encountering a dying Transformer who gives them a talisman. The TRF confronts Cade, only to be stopped by Bumblebee, Lennox, and Hound before letting them go.
Megatron, having abandoned his Galvatron identity, his remaining Decepticons, and the U.S. government learn of the talisman's value and reluctantly join forces to retrieve it and track Bumblebee to Cade's scrapyard. While the Autobots and the Dinobots fend off the Decepticons and the TRF, Cade, his assistant Jimmy, Izabella, and Sqweeks deal with a swarm of TRF drones sent after them. Cogman, a human-sized Transformer, appears and invites Cade to the United Kingdom to meet his employer, Sir Edmund Burton, who is connected to the Transformers. Burton asks another Transformer, Hot Rod, to find Viviane Wembly, a University of Oxford professor.
Burton explains to Cade, Viviane, and Bumblebee that Transformers have been living amongst mankind for centuries, their existence once guarded by a secret society called the Order of Witwiccans, of which he is the last living member. The talisman can lead to Merlin's staff, buried in a Cybertronian spacecraft under the sea. Viviane is revealed to be the direct descendant of Merlin's bloodline and the only one who can activate the staff. The authorities discover them, forcing the group to flee. Following clues, while evading their pursuers, the group heads to the Royal Navy Museum, where they commandeer the submarine , also a Transformer, to find the spacecraft while the TRF and the Navy SEALs tail them.
Burton contacts Seymour Simmons, both learning that Earth is Unicron, and the staff will drain the planet's life via an access point under Stonehenge. Locating the ship, Cade and Viviane find Merlin's tomb with Viviane activating Merlin's buried staff, awakening the Knights of Iacon. The TRF and Navy SEALs ambush Cade and Viviane to take the staff, but Viviane is forced to surrender it to Optimus, who has arrived to take the staff. Bumblebee, Cade, and Lennox engage Optimus, during which Bumblebee speaks in his voice for the first time after he is given a spare voice box, stirring Optimus's memories and freeing him from Quintessa's control. The Decepticons ambush Optimus and Cade and steal the staff, having aligned themselves with Quintessa. The Knights arrive to execute Optimus but yield to Cade and join the humans and the Autobots upon seeing Cade's talisman turn into Excalibur. Burton tries to stop Megatron from activating the staff at Stonehenge, only to be killed in the ensuing blast.
As Cybertron ravages Earth, using its thermal energy to reconstitute itself, the Autobots, Dragonstorm, and the humans launch a direct assault on Cybertron to defeat the Decepticons and Quintessa. Viviane removes the staff and stops the transfer. Optimus defeats Megatron while Bumblebee seemingly kills Quintessa. With both worlds saved but now attached, the Autobots leave Earth to rebuild Cybertron.
Meanwhile, Quintessa, who has survived, disguises herself as a human and approaches a group of scientists inspecting one of Unicron's horns and offers them a way to destroy him.