The upcoming season finale of Castle will feature Beckett (Stana Katic) and Castle (Nathan Fillion) getting ready to be married, when a roadblock gets in their way.

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Beckett and Castle soon discover that the former is already married to someone else, and has been for 15 years! As Beckett and Castle want to get married as soon as they can, she attempts to fix the situation, as seen in a sneak peek clip released by TV Guide Thursday. She learns that she was actually married to a man named Rogan O'Leary (guest star Eddie McClintock), to whom she pays a visit in the finale.

Beckett jogs Rogan's memory that they actually got married when they went on a drunken trip to Las Vegas, with the pair never filing for an official divorce.

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Rogan lovingly calls her "Kit Kat," but Beckett appears to be all business and she tries to get him to sign dissolution of marriage paperwork. Beckett is surprised to see that Rogan actually want to give marriage a shot, but she is obviously completely uninterested.

After Beckett turns him down, he agrees to sign the paperwork anyway, but on one condition. He teases that she has to do something for him first before he signs it, at which point the sneak peek clip cuts out.

The season 6 finale of Castle is titled For Better or Worse, and it will air Monday, May 12 at 10 p.m.

Castle season 6 airs Mondays at 10 p.m. on ABC.

Watch the sneak peek for the season finale of Castle here:

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Castle, Television, ABC