Beckett and Castle investigated the death of a young Wall Street broker on the latest "Castle."

In "The Greater Good," Castle (Nathan Fillion) and Beckett (Stana Katic) 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.

On the next episode, "That '70s Show," the team will break out their bellbottoms to channel the groovy '70s in an attempt to solve a murder case.

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