They say you never fully realize what you've got until it's gone, and truer words have never applied to Richard Castle and Kate Beckett.

In Castle Season 4, Episode 21, "Headhunters," Rick teams up with a new cop after tensions with his muse hit an all-time high. Unfortunately, his new partner, rough and tumble gang unit detective Ethan Slaughter (played by Firefly's Adam Baldwin), doesn't have his best interests at heart.

According to Nathan Fillion, Slaughter is a "Dirty Harry" type who won't hesitate to throw his colleagues under the bus. Unfortunately, Castle finds himself in a precarious situation when he accompanies the detective on one of his notorious, high-risk takedowns.

But on the upside, the experience with Slaughter brings Castle to some much-needed revelations.

"Castle comes to see the difference between someone having your back and someone throwing you to the lions," Nathan recently dished in the April 10 issue of TV Guide Magazine. "Slaughter doesn't care what happens to Castle."

But you know who does? Yep. Good ol' Detective Kate Beckett and the rest of his friends back at the 12th.

"With Beckett, Esposito, and Ryan, he has cops he can trust who are looking out for him."

Like we said before, you don't know what you've got 'til it's gone.

Source: TV Guide via SpoilerTV

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