These are recommendations made by Tropers for Castle fanfics, all of which have to be signed to stay on the page. Feel free to add a fanfic of your own to the list, but remember to use the template found here.
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Synopsis: I'm not a cop, he thinks. I'm... not a cop. And that means there are a lot of things I can't do. But it also means... I don't have to play by the rules.
Comments: Hands down the best look at the character of Rick Castle I have ever read. The story looks at a side of Castle that is rarely explored in fanfiction: the darker, capable and motivated individual who is very much Beckett's equal. While we see this side in the more serious episodes in the series, most fics tend to downplay just how capable he actually is. The story is brilliantly written, and it feels like a very natural extension of the series more serious moments. It lacks the sugary OOC characters that most Castle fics have.
I agree, this is brilliant. Easily the best characterization of Rick Castle I have ever seen. This is not the sappy Rick we see in many Castle episodes, but the man who was willing to risk his life for Kate's in the darker episodes, and the same man who gunned down 3XK in order to save her life. You need to read this.
Synopsis: A four-year-old, a mall, and finding something you didn't even know you'd lost. Sometimes all it takes is a chance encounter for everything to fall into place. AU with echoes of canon.
Comments: Great look at how the Beckett and Castle relationship could have worked out had they met earlier in her career. I love how the author captures the banter between Castle and Beckett as well as the genuine bond between Castle and Alexis without sugar coating the situation or characters.
Synopsis: Beckett breaks her leg and has to deal with the long, frustrating road to recovery. Luckily, she has her plucky sidekick by her side to take care of her. With any luck, he'll help her heal both her leg and her heart.
Comments: Extremely realistic yet fun portrayal of Castle and Beckett's relationship and how it would develop if Beckett had to depend on him even more than she already does.
Synopsis: A week after the end of Season 4, Kate and Rick talk about what happens next, and Martha becomes embroiled in a mystery of her own, finding she needs help from a certain ex-cop and her writer son. My usual hiatus-infill story! Read, enjoy, review! Now complete, just in time for Season 5!
Comments: A wonderful murder mystery set just before season 5. I love the fact that it actually is a mystery and feels like a wonderful filler for the show. Despite the fact that it was written before the season came out, it fits perfectly with canon, which ought to give you an indication of how well she captures the characters and tone of the show.
Synopsis: When Castle gets kidnapped while he is on a book tour, it falls onto Beckett the task of finding her partner before it becomes too late to save him, while they start to acknowledge what they feel for each other.
Comments: A very well written story, with tons of Continuity Nods from all the series (from Castle's CIA contact to Castle being compared with Steven Seagal). Special mention must be given to Castle's dreams while he is kidnapped, in which both him and Beckett become the main characters of several films and series - among them, the chapter "Shindig" from Firefly, the end of Quantum of Solace and, most memorably, Escape from New York, with Beckett as Snake.
Synopsis: Beckett mentioned that she met Castle years before the series, in a book signing. What would happen if, in that book signing, she crashed into a seven-year-old Alexis Castle and made friends with her favorite author?
Comments: brilliantly written, and great development of the characters. Things that are mentioned as past Noodle Incidents in the series get their time in this fanfic - Jim Beckett's problems with alcohol, Beckett's friendship with Madison, break-up with FBI agent Will Sorenson and problems with her mother's recent death, Castle's divorce and problems with Meredith - while Beckett and Castle become great friends at a time where she is still a police officer and he has yet to let his fame get up to his head. The presence of seven-year-old Alexis, an adorable girl, only makes it better, especially when she makes comments on the obvious attraction between her father and Kate.
Seconded by Positronic Brain. This is simply *the* Kalexis fic. Written from Kate's perspective, the author makes a great job of developing the younger versions of familiar characters and building a heartwarming story around them - and while Officer Beckett carries all the baggage of her TV counterpart, she still hasn't had time to build enough walls to keep the Castles out once they enter into her life. Of course, that also means she hasn't build enough walls to keep all her demons in...
Thirded by cozcat. Obviously an alternate reality fanfic, this is the fic that gets recommended if you say that you're new to the Castle fic world. It is a fascinating take on the characters as they would have been when they were younger, and something that you need to read at least once.
Synopsis: After discovering that Detective Kate Beckett's birthday is going to be soon, Richard Castle makes plans to make sure his muse enjoys her birthday away from work. Of course, when you take into account that there are still unresolved feelings between them and that they are going to be alone for a lot of time, you know it is not going to end as it started.
Comments: Brilliant and funny, well written, but the development of Beckett and Castle's relationship is a bit too fast, although given that they know each other very well it is somewhat believable.
Synopsis: What was going through Katherine Beckett's mind as she laid down on the grass, after being shot, while she heard Richard Castle telling her he loved her?
Comments: An interesting story, with each chapter being written from the point of view of a different character (as of chapter 19, Beckett and Castle have had 4 chapters, Alexis and Lanie 2 and other characters 1), with the most interesting being Chapter 10, where it explores the events of the Season 3 finale and the rest of that day from the point of view of the man that shot Beckett down. Funny, romantic and heartwarming at equal measures.
Synopsis: A recently made cop name Kate Beckett must work undercover as a stripper for an investigation. In the course of her work, she meets her favourite writer, Richard Castle, recently divorced from his second wife. They start a relationship that breaks when Kate's undercover work ends. What happens when they meet again, years later, as Detective Beckett discovers someone has been killing people using Castle's books as inspiration?
Comments: A story that has a strange start, with Rick seen as he was after he just got divorced for the second time and a less experienced Kate, only to later give us the series we love so much with the filter of Beckett and Castle knowing each other - in an intimate sense - before the series started.
Synopsis: Beckett's father was killed instead of her mother. She ends up as Castle's prosecutor for the naked horse ride incident, and he decides to tail her as a lawyer. Obviously, AU.
Comments: It takes a little adjustment to get used to Beckett as a lawyer, especially one who is so different to the fairly kick-ass cop that we know from the show. Once you get used to that, it is quite enjoyable, as it looks at how their relationship could have evolved from this standpoint.
Seconded by Positronic Brain. Kate's characterization is pure Fridge Brilliance. Being raised by a different parent, and affected by the way her father died, this Katherine Beckett has different hangups and traumas from her television counterpart. However, once Castle enters into the picture, we start getting glimpses of the strong, independent woman lurking beneath the surface. Also, honoring the title of the fic, lines and scenes from the television show are echoed by the story in entirely different contexts. Strongly recommended.
Synopsis: Alexis, after her rejection from Stanford, decides she needs to do more with her life than pad her transcript. Beckett and Castle help her organize a blood drive.
Comments: I know, I know, but the synopsis is purposefully deceptive. During the blood drive Kate accidentally stumbles into one of the greatest secrets in Castle's past - one so shocking that she can't even begin to fathom how he has managed to live every day with it. Beautifully written, with the twist tastefully handled and a last line that is a CMOHW for this troper.
Synopsis: A twenty-six year old police officer called Katherine Beckett get accidentally pregnant after a one-night stand with an unknown man, and has a daughter called Alexandra, or Al. Meanwhile, Richard Castle is a successful writer who lives with his girlfriend, actress Meredith. Five years after Al is born, Kate and Rick meet in a coffee shop, and become great friends.
Comments: A very interesting view on how the relationship between Beckett and Castle could have gone if it was Beckett who had a daughter, and on how the relation between Castle and Meredith could have gone if they had not had any children. It handles things quite beautifully, with the entire precinct treating little Al as a member of the combined 12th Precinct family and on how Castle's life would be without Alexis. Al is a lot like Alexis, but with blue eyes and brown hair. An excellent story!
Synopsis: Eighteen-year-old Kate Beckett meets twenty-six-year old Richard Castle in a coffeeshop. Richard has recently divorced Meredith, while Kate has yet to be struck by her life's disgraces. They slowly warm up to each other, and soon they start a relationship that will be tested by their personal and professional problems.
Comments: Although it starts in a strange moment, it is a fantastic read, because it shows how the relationship between Kate and Rick could have gone if they had bet before Kate's mother was killed, and how Rick's support helps not only her, but also her father, with all of them forming a strange family that nonetheless works great together.