The Way of the Ninja
S06, E18 (First Aired: Mar. 17, 2014)
When Castle and Beckett investigate the mysterious murder of a Japanese ballet dancer, they end up being targeted by a ninja assassin.
Bored with his success, celebrated mystery novelist Rick Castle teams with NYPD Detective Kate Beckett to solve the case of a copycat killer who re-creates murder scenes from Rick's novels. While solving the case, the two realize that their unorthodox partnership has its benefits and decide to continue working together. Combining his writer's intuition with her creative detective work, they investigate strange homicides in New York, all the while building a strong, if complicated, relationship with each other.