Adam Baldwin and Nathan Fillion, Castle Adam Baldwin and Nathan Fillion, Castle

Long before Nathan Fillion was solving crimes with Stana Katic on Castle, he was soaring through space on the short-lived cult fave Firefly. Now, for the first time, Castle is reuniting Nathan with one of his former co-stars on the ABC crime series. 

In an episode slated for spring, Adam Baldwin, who played Firefly's Jayne Cobb to Nathan's Capt. Malcolm 'Mal' Reynolds, will guest star as a detective named Ethan Slaughter. (How's that for a character name?)

Slaughter is described to me as a rough and tumble cop from the city's gang unit who lands a case that Castle is desperate to follow. Along the way, Slaughter shows Castle a dirtier side of police work that forces Castle to reconsider what he's willing to do to close a case. As the season nears its cliffhanging conclusion, expect this new alliance to place Castle in the line of fire like never before.

My sources tell me Nathan is over the moon about this fun nod to his sci-fi past. Nathan and Adam also co-starred together in Firefly's 2005 spin-off feature film, Serenity.

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