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In Space, Optimus Prime is captured and brainwashed by his sister Quintessa. Meanwhile, on Earth, Cade Yeager (Mark Wahlberg) and the Autobots are being hunted by the Transformers Response Force, led by a reluctant Major William Lennox (Josh Duhamel). Cade befriends teenage inventor Izzy (Isabella Moner) and meets Sir Edmund Burton (Anthony Hopkins), who reveals that the Transformers helped King Arthur build Camelot and that Merlin's staff is a Cybertronian device of great power that only Cade can retrieve because he's the last descendant of King Arthur. Burton hires historian Vivian Wembley (Laura Haddock) to help Cade. Megatron also wants the staff and joins forces with TRF to get it, but then betrays them. Optimus arrives on Earth with the remains of Cybertron in order to restore it by assimilating Earth. Bumblebee fights Optimus and breaks him free. Optimus then joins the Autobots and the humans to fight both Megatron and Quintessa, and awakens the Medieval Autobots for help. Cybertron is destroyed for good and both Megatron and Quintessa escape. Optimus reunites with the Autobots, who are pardoned by the government, while Cade hooks up with Vivian, adopts Izzy and leaves with them to find his daughter. In a post-credits scene, Quintessa reveals that Earth is actually Unicron and Cybertron's complete destruction has awakened him.
