Mickey's Adventures of The Mummy

In 484 CE, King Arthur leads his knights and men into a battle they can't hope to win. Having realized this, he enlists the help of the magician Merlin, who negotiates with a Cybertronian Knight. The Knight gives Merlin a staff wielding ultimate power; the twelve Cybertronian Knights combine to form Dragonstorm, who comes to Camelot's aid and wipes out their adversaries.

In the present day, Optimus Prime is absent while Transformers remain outcasts on Earth. Despite this, Transformers are increasingly arriving on the planet. Several governments formed the Transformers Reaction Force (TRF) to hunt and destroy Transformers of all factions; Cuba has become a safe haven for Transformers, with Topspin and Seymour Simmons hiding out there. In an area of war-torn Chicago, a group of kids stumble upon a crashed alien ship with a Cybertronian Knight inside. The kids are confronted by a TRF Walker, which is destroyed by Isabella, a survivor of the Battle of Chicago. Izabella and her Transformer companions, Sqweeks and Canopy, urge the kids to escape, but Canopy is shot by a TRF plane drone and dies. Shortly after, Bumblebee and Cade Yeager arrive and help the kids escape. Yeager finds the dying Knight, who gives him a metallic talisman. Barricade observes and informs Megatron that Cade has the talisman.

In the last few days, the Transformers' home world, Cybertron, has been approaching Earth. Optimus crash-lands on the planet, awakening him from stasis. Finding that Cybertron has been disassembled into pieces, Optimus confronts his maker, a powerful sorceress named Quintessa; she corrupts him, so he does her bidding — destroy Earth/Unicron, so Cybertron can live. Lennox, a former N.E.S.T. member who now works under the TRF, speaks with General Morshower, his former superior, about the presence of an ancient alien technology on Earth. This technology can help bring Cybertron into Earth's atmosphere, but only through the use of the staff given to Merlin. The TRF makes a deal with the reborn Megatron, who demands the release of his fellow Decepticons — Onslaught, Mohawk, Nitro, and Dreadbot. Megatron and his team go off to find Yeager.

Yeager hides out at a junkyard, which serves as a sanctuary for many of the surviving Autobots — Bumblebee, Hound, Drift, Crosshairs, Grimlock, Slug, Wheelie and others. While there, Yeager finds that Izabella has followed him back; Yeager tells her to leave but Izabella wants to stay and fight. Megatron's Decepticons arrive at the junkyard and attack the Autobots; in the ensuing chaos, most of Megatron's warriors are killed except Nitro and Barricade. Yeager is confronted by Cogman, a Headmaster, while an Oxford professor named Viviane Wembly is kidnapped by the Autobot Hot Rod. Both are brought to England and meet with Sir Edmund Burton, the last living member of the Witwiccan Order. Burton tells Cade and Viviane of the Transformers' origins, saying that the collision of Cybertron with Earth would spell certain doom. He tasks Viviane, the last descendant of Merlin, with obtaining his staff so they can prevent an ancient machine from being activated. Burton tells Yeager that, as a person defined by honor and dignity, he is the last knight. The talisman given to Yeager has wrapped around his arm by this point. The trio are forced to evacuate Burton's home when the TRF arrives.

With Yeager's help, Viviane delves into her father's past, and finds that they must take the H.M.S. Alliance into the sea to find the staff inside the ancient machine. Bumblebee, Yeager, and Viviane go to the Alliance with Cogman and Burton, who declines to go on the journey any further. The Alliance heads for the machine, but is intercepted by a TRF submarine. The Alliance ventures inside the machine, where Viviane and Yeager find a chamber of Cybertronian Knights, as well as Merlin's staff. One Knight, Skullitron, awakens and attacks them. The TRF intervenes and attacks Skullitron who falls to his death, while Viviane activates the staff and the machine rises towards the ocean's surface. As more Knights rise up to fight, Optimus, redubbed "Nemesis Prime" by Quintessa, intervenes, and takes the staff from Viviane with the intention of restoring his homeward. Bumblebee confronts and fights Optimus on top of the machine, as Yeager and Lennox struggle to step in. As Optimus is about to kill Bumblebee, he speaks, urging Prime to remember who he is. Optimus breaks free of Quintessa's control, but Megatron flies in and seizes the staff from Optimus. The Cybertronian Knights then attack Optimus for his betrayal, but Yeager — whose talisman becomes the Excalibur sword — stops the fight. The Knights yield to Yeager, who urges Optimus to protect the Earth once more.

At Stonehenge, Megatron, Nitro, and Barricade work with Quintessa and use the staff to activate the machine, controlling Cybertron. As the military intervenes, Megatron shoots Burton, who dies by Cogman's side. Optimus unites the Knights, and they become Dragonstorm. Using Lockdown's ship, they set off to stop Cybertron's collision with Earth. They raid Cybertron and battle against the Decepticons and Infernocons, Quintessa's soldiers. The Autobots and TRF soldiers destroy the Infernocons and guide Viviane and Cade so they can get to the staff. As Quintessa commences Cybertron's resurrection, the Autobots battle Megatron, who is defeated by Optimus and escapes after being kicked out by Optimus. Quintessa is destroyed by Bumblebee, also kills Nitro. Viviane removes the staff — stopping the collision of Cybertron.

The Autobots reunite following the battle. Optimus declares that his faith in humanity has been restored, and with Earth and Cybertron connected, they must work together to rebuild their worlds. Optimus sends a message to any surviving Transformers, telling them it's time to go home. The Autobots use Lockdown's ship to return to Cybertron.

In a mid-credits scene, scientists inspect inspect a part of Unicron. Quintessa, who has survived and disguised as a human, arrives and offers them a way to destroy the being.