A big episode of ABC's Castle is coming up that will feature Castle (Nathan Fillion) witnessing a murder take place, with the victim being a client who hired him.

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ABC has released a new plot synopsis for an upcoming episode of Castle titled I, Witness. The episode looks to be a serious one, with Castle being more involved in the case of the week than he might like to be.

"Castle is hired to uncover if a client's husband is having an affair, but the case takes a shocking turn when Castle witnesses the client's murder - by her husband," the plot synopsis reads. "But with no proof that the killing took place, Castle and Beckett [Stana Katic] must investigate to expose the truth."

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The I, Witness episode will air on Monday, Feb. 2 at 10 p.m. ET/PT on ABC.

It is interesting that the plot synopsis says that Castle and Beckett are both investigating the case, since the wording possibly suggests that they are working together again. It's possible that this could be the case that brings Castle back to the 12th Precinct.

In the mean time, Castle is still operating as a private investigator, with the next episode, titled Private Eye Caramba, having a more humorous angle considering the case will be set in the crazy world of telenovelas.

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

Watch the promo for the next episode of Castle below: