In the town of Storybrooke, Maine, all the residents are actually characters from fairy tales who were brought into our world by of a powerful curse. But they're all trapped and hold no memories of their past lives. Their only hope of salvation lies in Snow White's daughter Emma Swan who escaped the curse and is said to be the only one who can break it.
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Many fairy tales are woven together in Once Upon a Time and the world you get, in which Snow White is friends with Little Red Riding Hood and Cinderella, is a pretty interesting one. The familiar things about the stories stay the same, but the characters are given new sides to themselves and that makes them more exciting to watch.
One of the things I like most though is how the villains are offered as much screen time and attention as the heroes, so they're all equally fleshed out characters. Fairy tales are usually thematically centered around a battle between good and evil, but here you're shown both sides of the story. It becomes more like a clash of interests, where everyone only wants their own 'happy ending'.
With the world expanding to include characters from outside Grimm fairy tales in the new season I'm pretty excited for this show.