I don't know anything about the video game series that inspired this movie, although I understand it's better than this turgid piece of crap. This movie is a poster child for why video games shouldn't be turned into movies ... and somehow it dragged several excellent actors down with it.
Michael Fassbender plays both modern-day murderer Callum Lynch, and also Aguilar de Nerha of the Assassin's Creed Brotherhood in 1492. In the modern day, as in the past, the Assassins are fighting the Knights Templar: the Assassins fight for the freedom of mankind, the Knights believe in peace through the removal of all free will (yup). In the modern day, the Knights put Callum into a machine that allows him to relive parts of the life of his relative de Nerha, hoping to find the "Apple of Eden."
The plot only gets sillier, so I'll spare you. Fassbender apparently thought this project was good enough to be a producer. He's pretty much the only person in the whole thing who manages to bring a moment or two drama to the production. The others aren't trying very hard, but they have such a ham-fisted plot and lousy dialogue to deal with that selling this product is akin to selling shoes to snakes even if they'd been working at it. Some of the action sequences are mildly entertaining, although entirely unbelievable.
Overblown, over-long, and intensely ludicrous.