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First, I want to say I've never been entirely sure whether the title should be in all caps or not, but I'm writing it that way because it's stylized that way, but more importantly because, in the movie, it supposedly stands for "Retired, Extremely Dangerous." (But I guess it should still be pronounced "Red.") Also I need to say that the movie is based on a comic book miniseries that I don't recall having ever heard of. (I suppose I could have heard of it and forgotten about it, but I certainly never read it.) The movie came out in 2010, but I didn't see it until 2016, by which time there was also a sequel (which I hope to see eventually).
Frank Moses (Bruce Willis) appears to be just an ordinary, lonely retired guy. He regularly calls a customer service rep named Sarah Ross (Mary-Louise Parker), claiming he hasn't received his pension check (though actually he just tears it up, as an excuse to talk with her). One day, he tells her he's going to be in her city, and hopes they can meet in person. But before that can happen, a wetwork squad shows up at Frank's house, and tries to kill him. Instead, he kills them, and decides to find out who wants him dead, and why. But before he actually begins his investigation, he kidnaps Sarah, to keep her safe. He figures if he's been under surveillance, his enemies will know he cares about her, so she'll be in danger, too. He tells her he used to be in the CIA. At first, Sarah is understandably scared of Frank, but before too long she realizes he's telling the truth. And she soon becomes excited to be involved in what's going on, because it's just like the trashy spy novels she enjoys.
One by one, Frank begins contacting his old friends from the Agency, starting with his mentor, Joe Matheson (Morgan Freeman), who is now in a retirement home. Next is Marvin Boggs (John Malkovich), who's kind of crazy. And finally, Victoria Winslow (Helen Mirren). They all provide help in figuring out what's going on, and fighting back. The group also receives help from a former Russian agent named Ivan Simanov (Brian Cox), who has a romantic history with Victoria. The CIA agent who has been tasked with eliminating Frank and his allies is William Cooper (Karl Urban), though it's clear there's a lot that's being kept from him by his superiors.
And... I don't want to reveal any more of the plot. But the movie has a lot of cool action, a lot of humor, and... I dunno what else to tell you. It's all kind of ridiculous, but it's also pretty fun.