Production started on Paramount's Transformers: The Last Knight back in June, although very little has been released regarding the plot. Director Michael Bay has been posting photos and video from the set ever since filming started, without revealing any story details. Today we have word that Once Upon a Time star Liam Garrigan has been cast as the mythical character King Arthur, which essentially confirms months of rumors swirling around the sequel.

Mashable broke the news earlier today, with the actor surprisingly playing the same role in this movie, King Arthur, as he did in the ABC fantasy series Once Upon a Time. Back in June, plot rumors surfaced that the story centers on Optimus Prime returning to Earth in search of a mysterious artifact, which has ties to both King Arthur and the magician Merlin. This report also reveals that Santiago Cabrera, who had previously been cast, is playing Merlin.

Last week, new set photos surfaced which seemed to confirm the King Arthur rumors. The photos featured a number of actors wearing traditional medieval costumes, while riding on horseback during scenes filmed on the Scottish island of Skye. We don't know for sure how much of the story takes place in this ancient setting, but it seems that most of the sequel is set in modern day, with the sequel filming in Detroit, Cuba and several other locales. While other Transformers movies have featured flashbacks to different time periods, this would easily be the biggest time jump in franchise history.

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While we don't know how large or small his role is, Liam Garrigan joins a cast that includes Mark Wahlberg, Isabela Moner, Jerod Carmichael, Laura Haddock, Gil Birmingham and Anthony Hopkins. Original Transformers trilogy stars Josh Duhamel and Tyrese Gibson are also returning as Lennox and Epps, respectively. Michael Bay is directing Transformers: The Last Knight from a script by Art Marcum, Matt Holloway and Ken Nolan, who were all part of the Transformers franchise writers room.

Aside from playing King Arthur on Once Upon a Time, Liam Garrigan has also starred in TV shows such as Spotless, 24: Live Another Day, Strike Back and The Pillars of the Earth. He made his big screen debut in 2014's The Legend of Hercules as Iphicles. It's possible that we'll get our first look at Liam Garrigan in new set videos and/or photos that surface as production continues, but we don't know for sure when he starts filming his role.