Detective novelist Richard Castle (Nathan Fillion) met Kate Beckett (Stana Katic) of the New York Police Department because of a serial murder case and became very interested in her. Although Beckett strongly opposed it, Castle finally relied on his well-developed connections to "stay" in the New York Police Department and followed Beckett to solve cases in order to find inspiration for writing. The two people with different personalities and backgrounds gradually developed a tacit understanding in the constant bickering and friction, and became the best partners in name. However, Castle ignored Beckett's opposition and privately investigated the murder case of Beckett's mother many years ago, which once caused their relationship to be deadlocked. When Richard's "free-spirited" and "free and easy" style conflicted with Beckett's procedural and conservative style, arguments ensued and sparks followed. Is it dangerous or romantic?