When "Castle" returns, the team will break out their bell bottoms and platform shoes to channel the 1970s.

While the gang will be costumed, TVGuide confirms that the upcoming "That '70s Show" episode is more "grounded in reality."

Castle (Nathan Fillion) and Beckett (Stana Katic) will dress up in '70s era clothing to try to get the truth out of a witness who still thinks it's 1978. And unfortunately for Beckett, the witness has a pre-women's liberation view on women in the workplace, which forces Castle to take the lead on the case.

Following the disco-era episode, there will be only a handful of "Castle" episodes before the highly anticipated season finale.

TVLine confirms that Episode 21 will be titled "Law & Boarder," and 22 will be called "Veritas."

Then, the May 12 season finale, while not yet confirmed by ABC, will most likely be titled "For Better or For Worse," and will feature the wedding between Castle (Nathan Fillion) and Beckett (Stana Katic).

TVLine teases that some guest roles being cast for the upcoming finale include an ex-mob boss, a motorcycle gang leader, a stripper and a wedding officiant.

On the last episode of "Castle" before its hiatus, the team investigated the death of a young Wall Street broker.

In "The Greater Good," Castle and Beckett are trying to narrow down a small list of wedding invitees when they get a call to a crime scene.

A young Wall Street broker is found dead on his bed in his apartment, and the anonymous tip about the murder came from a woman using a pay-phone. Beckett and Castle think she's the killer, but it turns out that she's working for the U.S. Attorney's office with Gates' sister, Elizabeth.

The broker was a mole for an investigation into his boss, who has been offering a million dollar profit to his hedge hunters. The victim was seen with a gangster who may have ties to drugs, so the U.S. Attorney began investigating him.

Elizabeth is terse with Gates (Penny Johnson Jerald), as the two have a rocky history. It turns out that Gates arrested a witness with whom Elizabeth wanted to build a case.

It's then discovered that the victim made a $25 million wire transfer before purchasing a gun from the supposed gangster, who is his friend from Venezuela. It turns out the victim was adopted after his parents died in an earthquake in Venezuela. He was feeding the office bad information instead of cooperating with the investigation, and was going to escape back to South America.

He was caught by Elizabeth's aide and was killed.

Gates' sister then appreciates her finding out the truth and sparing her from a mistake that could have potentially ended her career.

In the end, Castle and Beckett edit their lengthy 600-plus wedding list to have a smaller, more intimate affair.

Watch the promo below for "That '70s Show," which airs Monday, April 21 at 10 p.m. EDT on ABC.