“The separation thing is getting really old.”

That line probably elicited a few groans from fed up "Castle" fans in episode 6 of the ABC series' eighth season. Castle (Nathan Fillion) and Beckett (Stana Katic) have been separated all season, but at least they have had a few scenes together each episode. In episode 6, "Cool Boys," a scheduling conflict means that Beckett has the hour off and Castle was left alone to find a new partner for the crime of the week. That partner would be Detective David Slaughter, played by guest star and Nathan Fillion's former "Firefly" co-star Adam Baldwin.

Slaughter, a reckless cop who works a network of shady contacts to take down gangs, paid a visit to the "Castle" universe in Season 4 for "Headhunters" and now the character is back to fill in for Beckett's absence -- the detective is MIA this week for some undisclosed reason relating to her secret project case. Slaughter enlists Castle's help in tracking down the criminals responsible for a seemingly impossible heist at a big office building. Castle agrees to help out, seduced by the promise of great writing material, but makes Slaughter promise not to hurt anyone -- the detective has a history of roughing up witnesses and suspects for information. 

Watch Castle agree to work with Slaughter in "Cool Boys" below:

Castle and Slaughter get to work and Slaughter is even hitting it off with Haley (Toks Olagundoye). However, the case takes a wild turn when one of Slaughter's insiders turns up dead. Then, Esposito (Jon Huertas) decides that Slaughter is a suspect and has him arrested! Luckily, Castle is able to track down the real criminals and goes with Slaughter to confront them. That leads to a tense standoff where the pair dance to a "West Side Story" musical number to distract their enemies while fighting their way out of the situation. In the end, Esposito and Ryan (Seamus Dever) show up to snap the cuffs on the actual culprits.

Watch Castle and Slaughter dance to "West Side Story" in "Cool Boys" below:

Later, Slaughter becomes the latest person to give Castle some advice about how to handle the situation with Beckett. He says to fight his way back into his wife's life -- whether she asks for it or not. 

“I lost my wife because I wasn’t man enough to put her needs ahead of mine,” he tells Castle.

Watch Slaughter give Castle some relationship advice in "Cool Boys" below:

Will Castle take Slaughter's advice? Fans will have to wait to find out. "Castle" airs Mondays at 10 p.m. EST on ABC.