In New York's meatpacking district, Beckett finds Castle waiting with two cups of coffee outside an apartment building. She complains about the cover art for his forthcoming Nikki Heat book -- mainly, the fact that Nikki's naked! She's not naked, counters Castle. She's holding a gun "strategically."
They enter an apartment bustling with uniforms and techs taking crime scene photos. Captain Montgomery breaks the situation down: 2-year-old Angela Candela was abducted from the apartment that morning. The Feds requested Beckett take part in the task force.
On hearing the name of the special agent in charge -- Will Sorenson -- Beckett reacts. When Montgomery asks if working with Sorenson is going to pose a problem, Beckett assures him it won't. Then she turns to see the girl's parents, Alfred and Theresa Candela, standing with FBI Agent Will Sorenson. The two detectives lock eyes and Castle immediately can tell there's history between them.
Sorenson hands Beckett a photo of Angela, an adorable 2-year-old girl clutching a stuffed bunny. Her dad, Alfred, got up with Angela at around seven that morning, planted her in front of the TV with a juice box and some cereal and headed into the next room, his art studio. He was listening to his iPod and didn't hear when she was taken.
In the kitchen, the Candelas show the investigators a jimmied window and a kitchen floor that's covered in dirt from the alley. When asked, the parents can't think of anyone who might do this. Sorenson notices a look on Beckett's face and steps close -- a little too close to be professional -- promising her that this case will "end better."
At the precinct, Beckett admits to Castle that she and Sorenson dated for six months. They met during a previous kidnapping case.
Ryan fills Castle and Beckett in on the parents. Married ten years, Theresa is a fund manager and Alfred is a small-time artist. Angela is their only child and was adopted. Alfred says he and Angela had the same morning routine every day. Which, according to Beckett, must mean someone either got very lucky or they already knew the routine. All of a sudden, her cellphone rings: The Candelas just received a ransom call.
In the Candelas' living room, Beckett, Castle and Sorenson huddle with Alfred and Theresa while they listen to a recording of the call. The caller demands $750,000 within 24 hours. It's the Candelas' entire savings. Beckett asks if there's someone who can help them get their finances together. Theresa's sister, Nina, is a CPA and can help.
Finding himself alone with Sorenson, Castle asks why Sorenson requested his ex-girlfriend for the task force. Sorenson claims he did it because Beckett is the best in the city. But Castle's not buying it.
Sorenson turns the tables, wondering why, out of all the detectives in New York, Castle chose to shadow Beckett. Castle quips that maybe it was fate. Just then, Beckett shows up with the Candelas' financial records. The ransom is indeed all the money they have. Sorenson figures whoever made the demand must know the Candelas well enough to know what they're worth.
Sorenson asks Theresa if she can think of anyone who might be holding a grudge against the family. Alfred suggests Doug Ellers. A former co-worker, Ellers was fired by Theresa and blamed her for his ensuing divorce, which resulted in Ellers' wife taking both his kids.
Beckett and Sorenson interrogate Doug Ellers. Sorenson plays a threatening voicemail Ellers left for Theresa, but Ellers swears he would never kidnap or hurt a child.
Afterwards, Castle suggests that Ellers is innocent. Sorenson disagrees. They argue until Beckett calls it a draw, saying that Ellers is probably innocent but when a child's life is at stake, you have to question everything you think you know.
Castle arrives home to find Martha in his office, "life coaching" one of her sad-sack clients. After the client leaves, Castle grabs her "Life Coach" name plaque and carries it into the kitchen, which he suggests is a better location for her coaching sessions. Castle mentions the kidnapping and admits he's feeling a need to give Alexis a hug. Perfectly understandable, retorts Martha, because parents and their children are like peas in a pod. This gives Castle an idea.
At the Candela apartment, Beckett notifies Sorenson that Ellers' alibi pans out. They share a moment of intimacy that culminates in a passionate kiss. When they part, they discover Castle standing there. "Justice never sleeps," he quips. Castle offers his new theory: Angela may have been kidnapped by her birth mother. As they look into it, Ryan gives them some relevant information: The birth mother, Lucia Gomez, recently went to the adoption agency requesting to contact her daughter's adoptive parents.
Beckett and Sorenson question Lucia Gomez. She says she doesn't regret giving up her baby. They show her the petition she filed with the adoption agency and she says it's a forgery. When they ask about the father, Lucia says he was in Iraq when she had the baby, but that he was "cool with it." When did he return from the war? Two months ago.
When Beckett and Sorenson approach birth father Juan Restrepo, he takes off running. Fortunately, Castle, Ryan, and Esposito are there to intercept him. When questioned by Beckett, Juan says he only wanted to see his daughter and make sure she was okay, since he never got a chance to see her when she was born.
Back at the precinct, they learn that Juan's alibi checks out. Frustrated, Beckett walks off, declaring that she's not going to lose this one. Castle learns from Sorenson that she's referring to the previous missing-child case. Even though they caught the kidnapper, the child was already dead. Just then, they get word that there's been another ransom call.
At the Candela apartment, they prepare the money for drop-off: seventy-five packs of hundred dollar bills placed inside a green backpack. The phone rings and the kidnapper demands that a civilian, not a cop, make the drop -- or they kill the girl.
Sorenson wants to make the drop, but Alfred won't allow it. He insists on following the kidnapper's instructions to the letter. Castle volunteers to make the drop. Over Sorenson's objections, Beckett gives the go-ahead. As much as she hates to admit it, Castle is their best shot.
As they're wiring Castle for the drop-off, a concerned Beckett asks if he's up for it. Castle is flattered and assures her it's going to be okay.
While in radio contact with Beckett and Sorenson, Castle carries the green backpack to a designated mailbox, under which he finds a cell phone. A text message directs Castle to a building on 1st Street. Ryan and Esposito tail him discreetly.
Inside the building, Castle finds a crowd milling in the lobby. As instructed, he leaves the backpack next to a shoeshine stand and walks away. Ryan and Esposito scope the crowd. Suddenly, Ryan spies an Asian male carrying the green backpack. Esposito closes in and tackles the guy. But he opens the backpack to find-- only newspaper. And then, he sees a girl with another identical green backpack. Ryan stops her and checks her backpack as well, only to find more newspaper.
At the apartment, they explain to the Candelas that the kidnappers posted an ad on Craigslist seeking participants with green backpacks for a "performance piece." However, Castle was able to slip the mailbox cell phone into the backpack with the ransom. So far, they've traced it to a twenty-block radius on the Lower East Side. Soon after, however, Sorenson receives word that they lost the signal. The kidnappers must have found the cell phone and taken the batteries out.
At home, Castle scrolls through photos he took of the Candelas' apartment, looking for a clue that they might have missed. Alexis sees a photo of Angela's stuffed animals on her bed and she and Castle reminisce about all the stuffed animals she used to have. Alexis notices another photo of Angela with the stuffed bunny, wonders if Castle thinks the bunny has something to do with the case. Castle looks at the photo, thinking--
Castle awakens Beckett as he comes into Angela's bedroom and starts searching for the stuffed bunny. He explains to Beckett that when Alexis was small, she couldn't sleep without her stuffed monkey, so much so they'd have to make sure to bring it on vacations. Now Castle can't find Angela's stuffed bunny. He surmises that the person who took Angela must have known her well enough to take the bunny, too. Beckett notes that they lost the cell phone's signal right after telling the Candelas that they were tracking it.
Beckett discreetly approaches Sorenson, asks for Theresa's sister's address. It's on the Lower East Side, the same area where they last tracked the phone. Coincidence?
They head to Nina's apartment and find Angela in a playground nearby, clutching her stuffed bunny as Nina watches over her. When Nina sees Beckett, Castle, and Sorenson approaching, she turns to run, only to find Ryan and Esposito closing in behind her.
When they return Angela home, an ecstatic Alfred runs and hugs his daughter. But Theresa stays at the table, stone-faced. She admits that she had Nina climb through the kitchen window and take Angela. Why? Because Theresa was planning to divorce Alfred, but didn't want to pay alimony or let Alfred have custody of Angela. The kidnapping would have proven Alfred an unfit custodian of their child, in addition to making it so that she didn't have to pay alimony.
At the precinct, Sorenson asks Beckett if she's willing to give their relationship another shot. But Beckett can't commit. Sorenson asks her to think about it and walks away. Castle confides to Beckett that he sees why it didn't work out: Sorenson and Beckett are too much like each other.
He then asks if she wants to grab a drink to celebrate closing the case, but Beckett turns him down -- she's got a date. When Castle asks with whom, Beckett quips that they call it a "private life" for a reason. As she walks off, she tells Castle that maybe there's more Nikki Heat in her than he thinks.