The one that got away.
The murder of a bridesmaid reunites Castle with his first true love, Kyra Blaine (guest star Alyssa Milano), who happens to be the bride of the wedding that never was. Beckett is at first surprised, having always pegged Castle to go for cartoonishly gorgeous pin-up girls and not anyone as 'real' as Kyra.
But as Beckett truly sees how much of a connection Kyra and Castle still have, it's hard for her to not be just a little bit jealous. Fortunately, both Castle and Kyra realize their time had passed. After Beckett and Castle close the case, Kyra goes through with her wedding. But not before telling Beckett, "He's all yours." Hint, hint, Detective!