Castle's latest case hits close to home for Alexis when her favorite singer, Hayley Blue, is found dead of a broken neck in an alleyway. She was hanging upside down from a ladder. A former drug addict, Hayley got clean and left her old band The Blue Pill. She'd been staying with Ian and Bree Busch, the producers of her first solo album. When Hayley went missing, the Buschs feared she'd gotten back into drugs.
Alexis feels horrible and wants to help with the case, but Castle orders her to get to school. Beckett is impressed. She says, "I'm so used to you acting like a 12-year-old all of the time; it's kind of refreshing to see you as a father." Then Castle asks if it makes her want him. Just like that, he's a 12-year-old again. Love it!
Hayley Blue had a stalker named Franco Marquez, who claims was at home listening to her brand new song, which has allusions to death. He tells Castle and Beckett he saw Hayley fighting with her guitarist Zack Metzger. Guess the one good thing about stalkers is they see everything. Zack tells Castle and Beckett they couldn't use the name "The Blue Pill" anymore, since Hayley owned it. Sounds like a motive to make her disappear, doesn't it? But Zack says he playing at a sweet sixteen party when Hayley was killed.
Hayley's sister, Sky, shows up at the morgue in a drunken rampage. Sky explains how Hayley was furious to find Sky pawning things to feed her own drug addiction. Haley wanted Sky to get clean, but baby sis found the request hypocritical after catching Hayley with a drug dealer named Tony. The dealer says Hayley wanted a gun, not drugs. There was powder residue on Hayley's hands but no weapon was ever found.
While listening to the lyrics of Hayley's final song, Threshold, Alexis tells her dad the word "death" symbolizes change. Alexis checks out the fan forums on the Blue Pill website. The rumor from band manager John McGinnis is that the Blue Pill is being reformed with Sky as their new lead singer. McGinnis had called Zack Metzger the day before Hayley died.
Beckett and Castle head back to the band's rehearsal space, where they find Zack and Sky fighting. They take Zack aside and tell him that they know he went missing during his sweet sixteen gig. Zack admits that he saw Hayley the day she died, at a coffee shop where she was writing new songs. He also noticed that she freaked out whenever she got a call from someone nicknamed "Death" on her cell phone. Yeah, that would freak us out, too.
Beckett agrees that for Hayley, "Death" meant change. And who symbolized the most change for her? Bree Busch. It turns out that Hayley's new producer had purchased the exact shade of lipstick that was later found painted on Hayley's lips in a parody of Hayley's music video. Bree admits that she'd tried to bribe Hayley with drugs but she refused them. They argued and then Hayley stormed out. Beckett doesn't buy it, especially after Castle notices a bullet hole hidden behind one of the panels in the studio.
When Castle catches Alexis listening to Hayley's last song again, he has a revelation. They may have the wrong killer. At the precinct, Castle and Beckett explain to Bree and her husband how Hayley's lyrics lead them to realize that Ian had forced himself on Hayley. Ian sent texts pretending to be Bree in order to get Hayley back to the studio. There, he and Hayley had struggled until he broke her neck. He set her body up in that alley way, hoping to make it look like her stalker had done it.
Things end on a bittersweet note as a drug-free Sky sings Hayley's last song, Threshold, at a memorial concert for her sister. Castle, Beckett, Martha and Alexis attend the candlelight tribute to the fallen Hayley Blue.