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While experimenting with magic when he was a kid, Sam accidentally cursed Dean. Now, Dean is forced to wear a spelled amulet constantly, or he'll turn into a random animal. For a little over a decade, he's learned to live with the curse, and has even found it useful in some cases, but he sure would be happier without it.
When he meets a witch named Castiel, he's offered a deal. Instead of assuming all witches are bad, Dean can spend a season getting to know him. If at the end of the season, Dean still thinks he's evil Castiel will send him away with his memory wiped of the whole experience. But if he learns that Castiel is not the monster Dean assumes he is, he'll lift Dean's curse.
It's an offer Dean can't bring himself to pass up.