Nathan Fillion stars in "Castle"

Writer's block -- it's that horrible feeling authors get when they just doesn't know what to write. It afflicts nearly all writers at some point in their career, but there probably aren't many who would resort to hunting down murderers as their cure.

But then, Richard (Rick) Castle is cut from a different cloth than most other novelists.

"Castle" follows the life of the titular novelist (Nathan Fillion) as he tags along with NYPD Det. Kate Beckett (Stana Katic) as she and her team investigate the most unusual and bizarre murders that the Big Apple can throw at them.

And bizarre they certainly are. In this season alone, the team has investigated a "superhero" vigilante, the murder of a ghost hunter and even an alleged zombie attack.

Having killed off the main character of his popular mystery novels, Castle teamed up with the homicide detective to get some inspiration for his next series. Along the way, Castle's unconventional way of solving problems and examining evidence turns out to be an asset for a team that definitely had its reservations about letting a civilian into its ranks.

The fourth season of "Castle" wraps up with its finale airing Monday, May 7, on ABC. Last season went out with a literal bang after Det. Beckett was shot in the chest by a sniper at her captain's funeral. As she lay in his arms, Castle told her that he'd fallen in love with her, a moment for which fans had long been waiting. It's a good bet that this season's ending will be just as eventful, with fans hoping for some juicy revelations about Capt. Montgomery's murder and a continuation of the romance blooming between the two leads.

Popularity of the show is so widespread that the novels being researched and written by Castle onscreen have been actually ghostwritten and released in the real world -- to surprisingly great success. The first two novels -- starring Nikki Heat, a detective based off Beckett -- both made appearances in the Top 10 of the New York Times best-seller list. The third book, "Heat Rises," debuted in the top spot, and a fourth book is in the works.

This crossover success comes as no surprise to fans. "Castle's" deep characters and murderous plot lines have a more comedic edge than the other crime drama to which it's most often compared, Fox's "Bones." While "Bones" isn't without its lighthearted moments, "Castle" is usually classified as a comedy-drama, or "dramedy," a combination that can leave the viewer laughing even while the team is dealing with the most serious of crimes.

It's a combination that has paid off. Since debuting in March 2009, the show has drawn consistently strong viewership numbers, and it has been nominated for several Emmy awards to boot.

Fillion is probably best known for his stint as Capt. Malcolm Reynolds on the now-defunct Fox series "Firefly." He stepped into Reynolds' shoes again in the big-screen continuation of the Space Western, "Serenity" (2005). He revives parts of his Reynolds role in "Castle" occasionally, like when he wore his "Firefly" costume in an episode of the ABC series.

"Castle" is probably Katic's biggest part to date. The Canadian-born actress has had small parts in a number of TV series, including "24" and "Heroes." Her role as Det. Beckett has definitely brought her into the spotlight, and it's a feat that didn't come easy. According to IMDb.com, she beat out 124 other actresses for the role.

The leading pair is also backed by a stellar cast of supporting characters, both in Castle's personal life and with the NYPD. On the home front, Molly Quinn plays Alexis, Castle's more-serious-than-dad teenage daughter, whose personal problems often mirror those of the case that her father is currently investigating.

Castle's mother, Martha (Susan Sullivan), also lives with the two of them. Sullivan is an accomplished actress who brings experience to the role, and is perhaps best known for her longtime role in the late '90s sitcom "Dharma & Greg."

Down at the station, Beckett's team is rounded out by detectives Javier Esposito (Jon Huertas) and Kevin Ryan (Seamus Dever).

After Capt. Montgomery's death last season, Capt. Victoria Gates (Penny Johnson) has joined the team as his more serious replacement. Johnson has a host of acting experience ranging from comedy ("The Larry Sanders Show") to drama ("ER") to nail-biting thriller ("24").

With a strong team behind them both at work and at home, the crime-fighting duo of Castle and Beckett is set to bring us lots of shocks and laughs with the finale on May 7.