Two new guest stars will appear on upcoming episodes of "Castle."

TVLine reports that "Eureka's" Salli Richardson-Whitfield will guest star in Episode 19, "The Greater Good." She will play Captain Gates' estranged U.S. Attorney sister.

After that, Jon Polito ("Millers Crossing," "Barton Fink") will play Harold Leone, a former consigliere to a mobster who has been missing since 1978. He will appear in the "That '70s Show" episode.

The ongoing theme of Castle (Nathan Fillion) and Beckett's (Stana Katic) impending wedding will also carry through the episodes.

While they are eagerly anticipating their nuptials, it is apparent that Beckett's engagement ring has only been spotted twice this season. Beckett's Stana Katic has a logical answer for why viewers rarely see the special ring.

"The important thing to remember is that Beckett is a detective, so she can't wear things that will identify her, she can't wear things like that on her hands that would distract from the conversation, that could be stolen, that could possibly hinder her when she's shooting or hand-fighting," she explained.

She added, "[The ring] is something that Beckett's very protective of, because she loves it, so on occasion, if it's like a big event or maybe a private moment outside of being in bed, she can wear it. But having it on display at the work place is not appropriate, especially as a detective."

On the latest episode of "Castle" before its week-long hiatus, Beckett went undercover as part of a sting operation.

In the episode "In the Belly of the Beast," Beckett (Stana Katic) gets a call on her day off from Gates (Penny Johnson Gerald), who tells her that she's wanted for an undercover operation.

Beckett impersonates a drug courier who speaks Russian, and has plenty of undercover cops to back her up.

During the sting, she gets kidnapped, and is offered a permanent job with the organization as an assassin. She and an associate from the organization go to perform a "test" job so Beckett can meet the ring's boss, a person named Lazarus.

She stages the hit, and the "victim" lets Castle (Nathan Fillion), Esposito (Jon Huertas) and Ryan (Seamus Dever) know that Beckett is still alive. They search for Beckett, but are unsuccessful.

Meanwhile, Beckett is able to identify where some of the laundered drug money is going while waiting to meet with Lazarus.

She finally comes face-to-face with Lazarus, who turns out to be Vulcan Simmons, a former drug lord in Washington Heights. He and his men torture Beckett for information, but she refuses to disclose anything.

They take her into the woods to be killed, which is when the original assassin shows up and saves her.

It turns out the depository for the drug money is a political Super-PAC, which is funding the presidential run of Sen. Bracken. Castle and Beckett now have a new mission: find a way to implicate Sen. Bracken.

But first, Castle and Beckett will try to outsmart a murderous ninja.

On the next episode, "The Way of the Ninja," "After a Japanese ballet dancer is inexplicably murdered, Castle and Beckett find that her secret life provides evidence of a ninja assassin, and now that ninja is after them."

Watch the promo below for "The Way of the Ninja," which airs Monday, March 17 at 10 p.m. EST on ABC.