Castle has come a long way in three seasons. Back in 2009, it was just a mid-season replacement with "that guy from Firefly." It's since developed into a Monday night staple with a host of fun characters that viewers have come to love.

Does the third season finale deliver on the stories and relationships that have been building for so long?

"Knockout" is the latest Castle story to focus on the series-long arc of the murder of Beckett's mother. Beckett is still making regular trips to the incarcerated hitman Hal Lockwood (Max Martini) who has knowledge of her mother's unsolved case. When he kills an inmate who also knows about the crime, he enlists the help of some shadowy figures to escape. As a result, Castle, Beckett and the homicide squad must track down Lockwood before he murders Beckett.

Before we turn a critical eye to Castle's final episode of the season, let's take a look at what it is and is not. Castle is a personality-driven show: the performances of Nathan Fillion (Richard Castle) and Stana Katic (Detective Kate Beckett) and the chemistry between these characters move the dialogue, if not the stories, forward.

Without the light-hearted and even flirty back-and-forth, there's not a whole lot to distinguish Castle from any mystery made in the last twenty years - and in fact, there's not much about it that stands out either. If you were to put Castle in a field with the likes of The Shield or The Chicago Code, it would undoubtedly be the "fun" show in the bunch. Castle is in line with Psych, Monk and Bones on the lighter side of murder mystery.

Until tonight, that is. With "Knockout," Castle takes a radical shift in storytelling and atmosphere. Gone are the puns, the jabs, and the good-natured sexual tension between Castle and Beckett. The finale strives to be pure hard-boiled Manhattan police drama, filled to the brim with professional killers, dirty cops and heavy-handed dialogue.

castle finale stana katic Max Martini

The shift is jarring to say the least. While Castle has had serious episodes in the past (almost all of which are in the same storyline), it's never gone as dark or as moody as the finale. For example, last week's episodes had the intrepid detectives and writer-sidekick catching the killer of a beauty pageant contestant - complete with a toupee-sporting Donald Trump stand-in. Earlier this year an entire show was devoted to the underground steam-punk aesthetic, with a Back to the Future DeLorean prop for good measure.

One of these things is not like the other.

A little dark flavor added to Castle's goofy veneer isn't necessarily a bad thing - in fact I was hoping for as much during the alien abduction episode a few months ago - but pouring this much drama into the last episode was a bad fit to say the least. I like just about all the characters on the show, from Beckett's bromancing fellow detectives to Castle's soundboard mother and daughter, but they simply haven't earned the moments that they were reaching for in the finale.

Nowhere is this more true than with Captain Montgomery (Ruben Santiago-Hudson). The third act sets up a huge dramatic revelation that does not sit well on the character's easy-going shoulders. The drama and performances feel forced to fit within the meandering story of Beckett's mother's murder, and Montgomery finishes the episode with a few action-packed seconds that feel totally out of place.

castle finale Ruben Santiago-Hudson

There's a moment where the homicide detectives and Castle are all investigating a crime scene, decked out in their "POLICE" and "WRITER" body armor, respectively. I couldn't help but think, "What is Castle doing here?" - which effectively sums up the feeling of the finale as a whole. Castle's Season 3 tagline was "solving murder has never been so much fun." Tonight, no murders were solved, and there was barely any fun to be had.

It isn't all bad. As convoluted as the primary plot is, it allows the viewer a believable motivation behind Beckett's tough-as-nails attitude. Those hoping for a romantic windfall for the two leads will be somewhat satisfied, though there's hardly time within the episode proper to explore their feelings. It sets up for a good reveal in the fall, and the blossoming romance between Castle and Beckett finally has the reason and the room to move forward.

Overall, "Knockout" feels like someone tried to rip Castle's characters out of their customary setting and drop them into an old episode of NYPD Blue. While the show has been circling its fun and witty stomping grounds for years and could do with a little shaking up, this much change, this quickly, is a poor move - and will feel even more strange when Castle inevitably returns to its comfort zone in the fall.

Follow Michael on Twitter: @ MichaelCrider