A deadly threat from Earth's history reappears and a hunt for a lost artifact takes place between Autobots and Decepticons, while Optimus Prime encounters his creator in space.
Having left Earth, Optimus Prime finds his dead home planet, Cybertron, and discovers that he was in fact responsible for its destruction. Optimus learns that he can bring Cybertron back to life, but in order to do so, he will need an artifact that is hidden on Earth.
Humans and Transformers are at war, Optimus Prime is gone. The key to saving our future lies buried in the secrets of the past, in the hidden history of Transformers on Earth. Saving our world falls upon the shoulders of an unlikely alliance: Cade Yeager (Mark Wahlberg), Bumblebee, an English Lord (Sir Anthony Hopkins) and an Oxford Professor (Laura Haddock).—ahmetkozan
Three years after Optimus Prime left Earth, the Autobots are still a world threat, at war with the Decepticons. Cade Yeager (Mark Wahlberg) is given a Talisman from a dying Transformer. However, the U.S. Military, and the Decepticons, find Cade and the Autobots, who are on the run once again. Cade meets a Transformer known as Cogman, and together with Bumblebee, they travel to London. They meet English Lord, Sir Edmund Burton (Anthony Hopkins) and head to a Cybertronian Knights' ship, deep underwater. Here they encounter Optimus Prime, now evil, and his master, Quintessa. Quintessa's intention is to collide Cybertron and Earth so Cybertron can begin again - so it's up to Cade; English professor Vivian Wembley (Laura Haddock); Sir Edmund Burton; Cogman; Bumblebee; and the rest of the Autobots to save the world.—ahmetkozan
In 484 A.D., King Arthur (Liam Garrigan) and his knights fight a losing battle against the Saxons. Elsewhere, Merlin (Stanley Tucci) approaches the Knights of Iacon, a group of Transformers hiding on Earth, to help win the war. They hand him an alien staff, before transforming together into Dragonstorm and turn the tide of the battle, but warn Merlin that a great evil will come for the staff. In the present, a year after the Hong Kong uprising, Optimus Prime crash-lands on Cybertron and meets his alleged creator Quintessa. She blames him for Cybertron's destruction and brainwashes him into helping to gain Merlin's staff, which can absorb Earth's energy to restore Cybertron. Optimus is renamed Nemesis Prime. Earth is revealed to be the slumbering Unicron, the ancient enemy of Cybertron and whose horns are emerging across the planet.
In 484 AD, King Arthur's (Liam Garrigan) wizard Merlin (Stanley Tucci) forges an alliance with the Knights of Iacon, a group of twelve Transformers who have hidden on Earth. The Knights give Merlin an alien staff and combine into a huge dragon to help Arthur triumph over the Saxons.
In the present day, most of the governments on Earth have declared Transformers outlawed, and the multinational Transformer Reaction Force (TRF) has been formed to destroy the alien robots. Despite the absence of Optimus Prime, who left the planet to search for his creator, new Transformers continue to arrive regularly; the newest ship to arrive crash-lands in the half of the ruins of Chicago, where it is found by a group of children. When a TRF Mecha confronts the kids, they are saved by Izabella (Isabela Moner), a survivor of the Battle of Chicago and her Transformer companions Sqweeks and Canopy, but Canopy is killed by the TRF in the process. Bumblebee and Cade Yeager (Mark Wahlberg) arrive and help them escape, but Yeager is unable to save the Transformer in the ship. Before he dies, the knightly robot attaches a metallic talisman to Yeager's body-an act observed by Decepticon scout Barricade, who reports this to his leader, Megatron.
On the far reaches of the Solar System, Optimus discovers that the Transformers' home world, Cybertron, now disassembled into pieces, is heading directly for Earth. He finds the being in control of Cybertron's movement, the sorceress Quintessa, who professes to be the maker he is searching for. The staff which the Knights stole from Quintessa and gave to Merlin. Quintessa wants to use the powers of the staff, she places Optimus under her control, dubs him "Nemesis Prime" and charges him with recovering it. Earth, she reveals, is actually Cybertron's "ancient enemy" Unicron and she intends to drain his life force so that Cybertron can be restored.Meanwhile giant horns start popping out at 6 locations on Earth. The Chinese call them "The Horns from Hell". NASA figures out that the horns are creating an inter-galactic dimension, through which the disintegrating Cybertron will enter in 3 days' time.
TRF member and former Autobot ally William Lennox (Josh Duhamel) brokers a deal between the TRF and Megatron, releasing from their custody a squad of Decepticons that will help Megatron recover the talisman from Cade. William is ordered by his seniors to do the deal to ensure that the weapon, the "staff" is found by Megatron so that the US military (who has Megatron under surveillance) can retrieve the weapon. William is also ordered to reveal Yaeger's location to Megatron, since Yaeger has the talisman which will reveal the location of the staff.
Megatron hunts Yeager down and follows him to his junkyard hideout in South Dakota, where he and many of the surviving Transformers are holed up. After the Decepticons retreat, Yeager is approached by Cogman, a Transformer envoy of British Lord Sir Edmund Burton (Anthony Hopkins), who takes him and Bumblebee to England to meet his master. There, Burton arranges a meeting between Yeager and Viviane Wembly (Laura Haddock), an Oxford professor. Wembly has just been kidnapped by the Autobot Hot Rod, using his long-term disguise as her Citroen DS.
Burton explains that he is the last living member of the "Witwiccan" order, an ancient brotherhood dedicated to guarding the secret history of Transformers on Earth. He also reveals that Viviane is the last descendant of Merlin, and that she must find and use his Staff to prevent the impending destruction of Earth by Cybertron. He reveals that Merlin was given the staff by the Transformer knights, and it was fused with his DNA, which means that only a direct descendant of Merlin can operate the staff and wield its awesome power. The staff was buried with Merlin's body and was protected by the 12 alien knights.
Fleeing the TRF, Yeager and Wembly follow clues left by the latter's father that led them, Bumblebee, and Cogman to take the submarine HMS Alliance into the sea to find the Cybertronian Knights' sunken ship, in which they discover the tomb of Merlin and the staff. Wembly activates the staff, and the ship rises to the surface; the TRF arrives to confront the group, but several Knights awaken and attack them. The attack is cut short by the arrival of the mind-controlled Optimus, and a battle between Optimus and Bumblebee ensues.
At its peak, the normally-mute Bumblebee finally speaks, the sound of his voice proving enough to shock Optimus free of Quintessa's control. Taking advantage, Megatron, who is revealed to be conspiring for Quintessa all along, arrives and steals the staff. As he flees with it, the Knights attack Optimus. However, Yeager, whose talisman becomes the sword Excalibur, stops the fight. Realizing that he is the last Knight, the Knights yield to Yeager, who urges Optimus to protect the Earth once more.
Megatron then delivers the staff to Quintessa, who begins draining the life force of Earth/Unicron via Stonehenge. When the British military intervenes, Megatron shoots Burton, who dies with Cogman at his side. Using a ship procured by the Autobot Daytrader, the Autobots arrive to join the fight, landing on Cybertron and battling against the Decepticons and Quintessa's Infernocons. Optimus and his Autobots, backed up by the Knights in their dragon form, vanquish their foes. Optimus sends Megatron flying from Cyberton back down to Earth, while Bumblebee shoots and apparently kills Quintessa. Viviane removes the staff, stopping Cybertron's destruction of Earth, destroying Cybertron in the process. Optimus declares that humans and Transformers must work together to rebuild their worlds and sends a message calling any surviving Autobots to come home.
In a mid-credits scene, scientists inspect one of the horns of Unicron, which is extending out of the desert. Quintessa, who has survived and is disguised as a human, arrives and offers them a way to destroy Unicron.
