ABC has released a plot synopsis for the highly anticipated Christmas episode of Castle, which features a run-in with the Mafia and a big moment for two supporting characters.

The network has released a synopsis for Bad Santa, an episode that promises a big moment for Lanie (Tamala Jones) and Esposito (Jon Huertas).

"When an ER physician is gunned down in the street, Castle [Nathan Fillion] and Beckett [Stana Katic] soon discover that the victim was a mob doctor to a Mafia family run by an old friend of Castle's," the synopsis reads. "Also, against the backdrop of Christmas, Esposito and Lanie reach a defining moment in their relationship."

'Castle' Christmas Episode Set To 'Change The Shape' Of Show Moving Forward?

The defining moment for Esposito and Lanie could be anything, from the two of them moving in with each other to the pair getting engaged.

Castle show runner David Amann recently spoke with TVGuide.com about the big Christmas episode, revealing that it is also a game-changer for Castle.

"We're looking at a bit of a game-changer for Castle at the end of the episode," Amann revealed, though he said this doesn't mean that Castle and Beckett are getting divorced. "The events of the episode will cause a shift...in Castle's life in a fairly significant way."

This also won't have anything to do with the mystery that kicked off the season.

Castle airs Mondays at 10 p.m. ET/PT on ABC.

Watch the promo for the next episode of Castle here:

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Castle, Television, ABC