Castle and Beckett are called to a wedding to investigate the death of a bridesmaid, and Castle is stunned to discover that the soon-to-be bride is his old flame, Kyra (guest star Alyssa Milano) — "the one who got away." Complications ensue as Kyra, caught off-guard by Castle's reappearance in her life, starts to wonder if she's marrying the right guy. With the nuptials forestalled and the entire wedding party under suspicion, Beckett questions Castle's ability to be objective about the case. But when Beckett begins to suspect that Castle and Kyra might rekindle their romance, she loses her ability to be objective as well.
