Nathan Fillion

The Castle wedding is (hopefully) drawing nigh, which means one thing: The battle to be the best man is on!

As you can see in the exclusive first look at Monday's episode (10/9c, ABC) below, Beckett (Stana Katic) has already chosen Lanie (Tamala Jones) to be her maid of honor. However, when Ryan (Seamus Dever) and Esposito (Jon Huertas) learn that Castle (Nathan Fillion) has yet to select a best man, you can see their competitive wheels start turning.

Finale Preview: Get scoop on 39 must-watch season-enders

"Ryan [wants] to have nice time at the bachelor party," Dever recently told TVGuide.com of his character's approach to winning the job. "He knows a great place with a great wine list. He's going to be a classy best man, where Esposito is going to be like, 'Yeah, I'm your frat boy!'"

So will Castle pick one of his precinct buddies, or will he pick a neutral third party? Unfortunately for both men, Castle may be clueless to the competition that's going on around him. "The whole time they're jockeying for favor, all of their efforts are wasted because he's not even aware of what they're trying to do," Dever teases. 

Castle airs Mondays at 10/9c. Check out the sneak peek below and watch full episodes of Castle here.