The moment Castle fans have been waiting for since the show debuted in 2009: Rick Castle and Kate Beckett are finally getting married in the Season 6 Finale episode titled "For Better or Worse."

Tonight's episode will start at 10 p.m. ET and can be viewed online through live stream via the link provided below.

The official summary to Monday's episode was released as follows:  

"A secret investigation conducted by Beckett related to her mother's murder goes awry when a man she has been surveilling is killed. When evidence links Beckett to the victim, she and Castle become the targets of a manhunt that puts both of them in grave danger," according to the outline.

However, things will not go smoothly - of course - and spoilers have revealed that a big surprise will hit the show tonight in the finale.

Apparently it will be discovered that Beckett (played by Stana Katic) is secretly married to a bartender named Rogan O'Leary, and has been for the past 15 years after they met each other in Las Vegas when she was intoxicated that night.

Now in the rush to get married, Beckett has to terminate the marriage to O'Leary before she can marry her soul mate Castle (played by Nathan Fillion of "Firefly") in just three days.

Season 7 of the hit ABC series is expected to return this fall (most likely on September 22, 2014).

"Castle" Season 6 Finale airs tonight at 10/9c on the ABC network. "Castle" can also be watched online through live stream by clicking here.