Beckett's initial relationship with Castle was strained because she viewed him as immature and reckless.She suspected (correctly) that Castle was using his "research" as an excuse to pursue her, and only tolerated his presence at the request of Captain Montgomery.

In "Food to Die For", Castle and Beckett discuss their victim, killer and the girl caught in between.

Castle says that the girl should have gone with the one she obviously truly loved, and Beckett argues that she saw the logic in staying with the one that seemed responsible and more dependable at that time, as opposed to the one with a history as a playboy that makes you feel good for a while, but has a risk of just letting you down eventually.

But it's too late: Castle has already planned to go to the Hamptons with another woman: Gina, his ex-wife, with whom he's rekindled a relationship.

Castle explains this, believing that he's gracefully stepping aside, but this crushes Kate, who waited too long and took an emotional risk for nothing.

While hotly denying it, she is clearly grief-stricken in "When the Bough Breaks" when it seems that Castle will be ending their partnership now that he's finished writing Heat Wave.

In "A Rose for Everafter", Castle reunites with an old flame.

Castle, for his part, assumes that she's still with Demming and has procrastinated about contacting Beckett again because of his unrequited feelings and a sense that things might not be the same.

They are forced together when the detectives arrive at a murder scene to find Castle standing over a dead body while holding a gun.

Beckett then develops a romance with a handsome young detective, Tom Demming.