Eliska Sokel, an illegal immigrant from Eastern Europe, is found beaten to death in a sewer manhole, and her lonely apartment throws up a possible clue; a photo of a mother and son, with the mother's face viciously scratched out. Investigation reveals them to be the Talbots, a family who lived in the luxurious apartment the victim worked in — until recently, when Mrs. Talbot raised a complaint about her that led to Sokel being fired. The relationships between Sokel and the Talbots soon reveal to be more complicated, however, with an ugly secret in their shared past waiting to be revealed.
Meanwhile, the launch of Heat Wave is approaching, but Castle has been approached with an offer to write a series of novels about "a certain British secret agent", a dream job of his — and one that would necessitate him walking away from his 'Nikki Heat' series and, by extension, Beckett. This leads to tension between the two as Beckett finds herself less happy about this eventuality than she'd anticipated.
The Castle episode "When The Bough Breaks" contains examples of:
- Be Careful What You Wish For: Beckett has clearly been counting down the days until Castle is out of her life — until the prospect of it actually happening for real starts looming, at which point, despite her protests, she is clearly unhappy about it. There's also an element of this regarding Nikki Heat — she hasn't enjoyed being the basis for the character, but does not like it when Castle doesn't seem interested in writing about her further. There may be parallels here with her relationship with Castle. It's hard to tell.
- Bittersweet Ending: The case is this. Eliska was killed before she could rightfully regain custody of her biological son. But her husband will be allowed custody of his son and will be able to reconnect with him.
- Break the Cutie: Both the victim of the episode and her husband, in the backstory, as well as Melissa Talbot, who comes across as a fairly pleasant woman, but by the end of the episode finds out that her biological son died young of an incurable disease, her husband switched their son with another baby in order to avoid them having to go through that heartbreak and then murdered the other baby's mother for wanting her son back. She's also initially seeming likely to lose custody of the boy she's been raising as her son for several years. However, Melissa and her 'adopted' son's biological father do have an amenable meeting at the end in which he makes it clear he's not going to take the boy away and they trade information about both of their sons.
- Cannot Spit It Out: Castle, twice. The first time, he derails a warm-and-fuzzy romantically-charged moment between himself and Beckett to bring up a thought he'd had about the case, leaving Beckett visibly suffering from Mood Whiplash. The second, when it looks like he and Beckett are parting ways for good, he ends another potential romantic moment by holding out his hand for a handshake. Oh, Castle.
- Death of a Child: Eliska and Teodor's son Martin died of an unspecified illness (later revealed to be Niemann-Pick syndrome) three years prior to the episode, which eventually caused them to divorce. When Dr. Talbot admits to killing Eliska, he reveals that he switched Martin and Zane as babies because he didn't want them to deal with Zane dying so young.
- Embarrassing Alibi: Subverted. Eliska Sokal is found murdered, and one of the suspects is Dr. Cameron Talbot, who was seen arguing with the woman. She had worked at his apartment complex, and she had been let go after Talbot's wife complained to the staff that Eliska had shown unhealthy interest in her son, Zane. Dr. Talbot claimed that he had had an affair with Eliska and that the argument was about him trying to offer her money to keep it silent, and that at the time of the murder, he'd been in his office. The truth is far more terrible; Talbot had stolen Eliska's newborn son when his own was diagnosed with an incurable genetic disorder, swapping the two children. Eliska had figured it out, and was trying to prove that Zane Talbot was, in fact, her own child. There had been no affair. Talbot had lied about that to make it seem like he had an embarrassing secret to keep from his wife that might keep the police from looking in the right direction.
- Face of a Thug: The victim's ex-husband Teodor has an intimidating aspect and is initially presented as a suspect with suggestions of spousal abuse. He turns out to be a perfectly nice man, heartbroken by the loss of his family and who never laid a hand on his ex-wife, her only leaving him due to the pain of losing their son.
- Foreshadowing: The Switched at Birth twist is hinted at with the picture Teodor shows Beckett and Castle having baby "Martin" with blond hair and blue eyes while Teodor and Eliska both have dark hair and brown eyes. "Zane" also has brown hair and brown eyes while the Talbots both have blond hair and blue eyes.
- Grief-Induced Split: Eliska lost her baby boy to an incurable disease. She left her husband out of grief, and he is still heartbroken over the loss of his family.
- Moment Killer: Almost every warm-and-fuzzy moment between Beckett and Castle in the episode is spoiled by something:
- Castle ruins it himself at the book launch when he brings up a thought he's had about the Body of the Week. The attempts to get the moment back end up with both of them fighting like teenagers and storming off.
- Captain Montgomery and Paula, Castle's agent, ruin it by telephone at the end when Castle and Beckett are saying their heartfelt goodbyes to each other.
- Mood Whiplash: Castle and Beckett seem to be having a warm fuzzy moment at his book launch, when Castle suddenly brings up a thought he had about the current murder they're working on. Beckett is left visibly stunned.
- Pun:Castle: I almost bought a Russian bride once. You know, a Czech-mate?
- Removed from the Picture: Castle finds a picture of Zane and Melissa Talbot (with the latter's face scratched out) in Eliska's apartment. Given the reveal about Martin and Zane being switched at birth, it's likely that Eliska thought Melissa was in on the switch.
- She Cleans Up Nicely: Beckett, at the book launch. Lampshaded in pretty much these exact words by Montgomery.
- Shown Their Work: The postcards in Eliska's flat really are pictures of Prague. Castle and Becket pronounce her name with a stress put on the second syllable: e-LIS-ka, but when her ex-husband talks about her, he pronounces the name exactly as a Czech would do with a stress on the first syllable: EH-lish-kah.
- Switched at Birth: Dr. Talbot switched the name tags on Zane and Martin's cribs after they were born, taking Martin to be raised as Zane and vice versa. However, Eliska gradually worked out what had happened after seeing Martin/Zane, and had planned to expose Talbot before he murdered her.
- Writing Around Trademarks: It's pretty clear who the "certain British secret agent" Castle is invited to write is, but it's strategically framed as Castle refusing to say the name or have it said around him to avoid jinxing the deal.
