Alex (Chyler Leigh) and Kara (Melissa Benoist) are thick as thieves, but that bond will be tested during this year's crossover between Arrow, Supergirl and The Flash. (DC's Legends of Tomorrow is too busy chasing deadly magical creatures across time and will not participate this year.) In a sneak peek at the three-part event, titled "Elseworlds," Kara finds herself behind bars and at the mercy of an alternate version of her sister Alex who views Supergirl as a danger to society.

"Alex, please. I know the world looks one way to you," Kara says in the video. "We're sisters." However, Alex ignores these pleas and slams the door shut on Kara, signaling troubling times ahead for the sisters.

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"Elseworlds" will send our heroes into an alternate universe where history has been rewritten, so expect lots of weird things like Oliver (Stephen Amell) and Barry (Grant Gustin) swapping bodies. Raise your hand if you're still perturbed at the image of Oliver kissing Iris (Candice Patton)!

A bunch of familiar DC characters will drop by including Jay Garrick (John Wesley Shipp), Superman (Tyler Hoechlin), who will debut a nifty new black suit; Lois Lane (Grimm's Elizabeth Tulloch); Mrs. Freeze (Cassandra Jean Amell); and The Monitor (LaMonica Garrett). Plus, Batwoman (Ruby Rose) will finally make her Arrowverse debut.

Everything We Know About the Arrowverse "Elseworlds" Crossover

Look out for the three-part crossover which begins Sunday, Dec. 9 at 8/7c with The Flash, which will air in Supergirl's regular time slot. "Elseworlds" continues with Arrow on Monday, Dec. 10 at 8/7c and Supergirl will close things out in The Flash's typical time slot on Tuesday, Dec. 11 at 8/7c.

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