'Castle' Season 8 Spoilers: Stana Katic, Ann Cusack and a Russian mission

"Castle" Season 8, Episode 11 ("Dead Red") airs this Monday on ABC. This week's new episode comes a night after a Sunday special, titled "Witness for the Prosecution." Stana Katic and Nathan Fillion is joined by Ann Cusack, who plays the role of Rita, Rick's stepmother. A Russian diplomat is murdered, so the characters have to deal with some Russian officers. Fans believe there will be some action-packed spy work, because Rita is back.

Spoiler alert: This update contains "Castle" Season 8 spoilers. Read on if you want to find out what happens next to mystery writer Rick Castle (Fillion), Captain Kate Beckett (Katic) and the rest of the characters.

In the Sunday special -- a first for the ABC dramedy series -- Beckett concludes that a lawyer associated to Locksat, Caleb Brown (Kristoffer Polaha), has a sense of justice about him. He wants to do something good, the 12th Precinct captain tells Vikram (Sunkrish Bala). Together, they manage to free an innocent woman from jail, after staying there for five months for a crime she did not commit. This is not the last time that Brown would appear in "Castle" Season 8, since it was established that Beckett and Vikram will continue digging up information about his real links to Locksat.

This Monday, Castle and Beckett find themselves learning more about international crime and foreign politics. It is not clear whether Rick and his stepmom Rita will share the screen together. Rita was last seen in "Castle" Season 8, Episode 2 ("XX"). Rita's husband and Rick's father, Jackson Hunt (James Brolin), was last seen in Season 6.

"A Russian diplomat's son is killed, leading Castle and Beckett to learn about international crime and foreign politics as Castle is tasked with keeping a Russian security officer busy," reads the new episode synopsis from ABC. Aside from Ann Cusack, the guest stars this week are Nikita Bogolyubov as Grigory Mishkin and Endre Hules as Pavel Oborin. Watch the "Castle" Season 8, Episode 11 promo video below. Live stream is also available via ABC Go.

Meanwhile, Stana Katic recently reacted to a fan commenting her show's connection to "X-Files." The comment came after Katic tweeted her "smart interview" comment on Gillian Anderson's Net-a-Porter feature. Incidentally, a number of people running "Castle" are from "X-Files." Rob Bowman, who directed many of the biggest "Castle" episodes, was a TV and film director of "X-Files." Here is the tweet exchange:

WATCH: 'Castle' Season 8, Episode 11 promo video (ABC on YouTube)