We've been so pumped for Monday night's much-anticipated Firefly reunion that we've forgotten to take note of all the wonderfully juicy Castle and Beckett spoilers for Castle Season 4, Episode 21: "Headhunters."

Here's what we know so far: Castle begins working with tough guy Ethan Slaughter, a detective from the city's gang unit (played by Adam Baldwin), after tension with Beckett reaches an all-time high. At first, the grass in Slaughter's yard appears greener — until Castle realizes that his new partner doesn't have his best interests at heart.

Although Beckett's worried about her former partner's safety, she's too hurt by his recent change in behavior to do anything about it. However, she does consult her therapist, who opens her eyes to a new possibility: Maybe Castle moved on during the seven months Beckett spent healing (but we know better!).

Meanwhile, Castle has a revelation of his own. Life on the road with Slaughter isn't all it's cracked up to be. In fact, his experience with the new detective teaches him about what matters most: People who have your back, always.

Sounds to us like Castle has a case of "don't know what you've got 'til it's gone."

Check out the sneak peek clips from the episode below:

Castle surprises Beckett at the precinct (bearing Starbucks coffee, no less!), but he's not interested in catching up with the detective. However, he is interested in another detective. One by the name of Ethan Slaughter. Talk about a snub! Poor Becks. 

 

Later, Castle swings by the precinct again, this time with an entourage and a bloody nose. Beckett's clearly concerned about Castle's safety, but the mystery writer is too enamored with his new muse to notice. 

Credit: Access Hollywood Photo: Spoiler Clip of Castle Season 4, Episode 21, "Headhunters": Beckett Fears for Castle's Safety

 

Beckett seeks counsel from her trusted therapist, Dr. Burke. Will these new revelations inspire her to take action and go after what she really wants? Only time will tell. 

 

Original Air Date: Castle Season 4, Episode 21: "Headhunters" airs on Monday, April 16, 2012 at 10 p.m. ET on ABC. 

Credit: YouTube Photo: Castle Promo for Season 4, Episode 21, "Headhunters": Rick Gets a New Partner!

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