A serial killer from ABC's Castle's not so distant past is returning in the upcoming two-part episode.

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Michael Mosley, who played serial killer 3XK through several episodes of Castle, is slated to reprise his role in February's two-part episode, according to a new report by Entertainment Weekly.

The report reveals that Dr. Kelly Nieman (Annie Wersching), the plastic surgeon thought to be a disciple of 3XK, will also be returning in these episodes.

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"There's a murder that our cops are investigating at the beginning of the episode that seems to connect to [Dr. Nieman]," Castle showrunner David Amann told the publication recently. "That puts us on what we think is going to be a fun, dramatic journey for the audience as we try to solve that murder and figure out what her involvement is in it, if any."

Meanwhile, Castle (Nathan Fillion) is busy becoming a private investigator, working parallel to the 12th Precinct that let him go in the winter finale. However, he will likely be investigating his vanishing at the beginning of the season in the two-part episode, according to Amann.

Amann previously told TVLine that the vanishing plot would "probably" pick back up again in the two-parter of episode 14 and 15, slated to air in February.

Castle returns from winter break with a new episode on Monday 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