The episode begins in a small hotel room where a woman is taking shower. She is shocked to see blood pouring out of the shower instead of water. A man goes to check the water tank and finds a dead body floating in the tank. Kate is in DC for an interviewing at the Attorney General’s office. At home, Alexis tells Castle that her professor hasn’t received the check for her Costa Rica trip. Castle tells her that he doesn’t want her to go to study in the rainforest for 6 weeks. Alexis knows that this is about Paris. She explains that she doesn’t want to live her life in fear.
Castle reluctantly agrees to pay for the trip. His phone rings and he gets called to the recent crime scene. Ryan and Esposito brief Kate and Castle about the crime. They tell them that maintenance found a dead body in the water tank. The victim is a Caucasian female. Lanie tells Kate that there is evidence of blunt force trauma to the head but she cannot be sure till the time she finishes the autopsy. There are traces of blood and hair on the ventilation fan. There is blood on the hand railing and also a bloodied partial print. The resident manager identified the victim as Crystal Sky. She checked in ten days ago to a room on the 18th floor and paid in cash. There is no other id found.
Kate tells her team that they should talk to the other residents because she is sure that Crystal must have made friends in the past ten days. They talk to a guy, Ziff, and learn that Crystal is from Texas. Another resident woman tells the detectives that Crystal was a prostitute. Esposito tells Kate that CSU found no prints in the room; it looks as if it has been wiped out. Later, Ryan arrives with more information on the victim. Her actual name is Erika Albrook and she is an Honor student from Harvard. Kate and Castle talk to Erika’s parents, and learn that Erika was supposed to be in Europe with her friends. The mother tells Kate that Erika would never stay in a place like that.
Later, at the precinct, the team checks the security camera footage from the elevator. They find that Erika was the last one to go to the roof, and it appears that she was making sure that no one is seeing her or following her. Kate figures out that the person who killed Erika must have taken the stairs. Ryan tells Kate that Erika used her credit card at an expensive restaurant called Margos, and she split the bill with a girl named Talia. Ryan tells them that according to the noises that came from Erika’s room, it is clear that she was a prostitute. In the morgue, Lanie tells Kate and Castle that she found no signs of sexual activities on Erika.
However, there are defensive wounds on her body and around the neck. She also tells them that there were several blows made to Erika’s head. Kate and Castle go to Erika’s room and find a cd that played the “noises” the other residents used to hear. They talk to Ziff once again. Ziff tells them that he used to spy on Erika through a hole behind the closet. He says that there were only noises, but nothing used to happen. Erika used to wear her headphones and continuously type on her laptop. The duo concludes that Erika only wanted people to think that she was a prostitute. Kate also points out that there was no laptop found in the room. Ryan and Esposito question Talia.
Talia tells them that she saw Erika arguing with a guy, who was sort of threatening her. She tells them that she tried talking to Erika after the guy left. She says that Erika panicked on seeing her and she begged her not to tell anyone that she was here, else she could get into a lot of trouble. Talia tells the duo that Erika seemed to be on some sort of a mission. She also describes the man Erika was arguing with. They get the sketch made. At the precinct, Castle and Kate theorize that Erika was a hacker. She was hacking into something using the hotel’s WiFi connection. Kate tells Ryan and Esposito to work on this lead.
Gates calls Kate to her office and tells her that she has been shortlisted for the post in DC. Gates is really happy for her and tells her that this could be her last case as a homicide detective. Kate goes to Laine for some advice. She still hasn’t told Castle about her job in DC. Esposito and Ryan manage to track down the IP address Erika was hacking into. It belongs to a law firm. Castle and Kate go to the law firm and meet Jessica Banks. Banks tells them that she hasn’t seen Erika. She asks her partner, Mitch, if he knows anything about Erika. He says no. They talk to the IT guy and find out that Erika did hack into their servers. Ryan arrives with another lead.
He tells them that Erika used the hotel’s pay phone to make her calls, and it seem she called a guy named Ian, who used to work as a lawyer at the law firm, Banks and Bower. Ian was fired six months ago. Ian is the same guy who Talia saw with Erika. They go to Ian’s house but they find him dead. Lanie tells them that this was a staged suicide. The killer strangled Ian first, and then hanged him. Lanie also tells them that Ian was murdered 24 hours before Erika. Ryan finds Ian’s laptop and sees that Ian and Erika were chatting, and Erika had asked him to meet her at the hotel rooftop. Castle points out that the timelines don’t match because the chat was sent around 10 pm the day Erika was killed, but according to Lanie, Ian was already dead by then. So, it was the killer who lured Erika to the rooftop.
At the precinct, Ryan tells the team that Erika’s best friend, Pam, was an intern at the law firm and she died in a car crash last summer. He tells them that autopsy indicates that Pam was intoxicated at the time of the crash, but Erika never believed it. According to Erika’s statement to the cops, Erika did not believe that Pam would drive when she was drunk, and also Pam never drove wearing heels. So, all of this is connected with the law firm. They figure out that the firm is trying to cover up something. They then put everything together, and Kate and Castle figure out that Erika hid the laptop in the basement before she went to the roof. Esposito and Ryan go to find the laptop.
Ryan tells Esposito that his wife is pregnant. Esposito is happy for his friend. They find the laptop. At home, Castle learns about Kate’s trip to DC. He is upset that Kate hid it from him. He knows that if she gets the job, their relationship would end. The next day, at the precinct, the team learns that Erika was looking into the firm’s accounting department; especially Ian’s billable hours during last summer. This is the time when Pam met with an accident. They see that around the time of Pam’s death, Ian got a call from one of the firm’s clients. Kate thinks that someone else was driving Pam’s car. The person, who crashed, didn’t want to be blamed for Pam’s death.
So, he put her behind the wheels, fled the scene and called his lawyer. The tech woman finds out that Ian billed the same client for a game of golf. The client turns out to be Colin Rigsdale, who is being groomed for the State Senate run. Kate goes to talk to Colin. She tells him that they know he met Pam at one of the firm’s parties, and he was involved with her. Erika started investigating her friend’s death, and she too ended up dead. Banks does not let Colin talk much. Kate returns to the precinct and tells her team that she is sure that it is Colin. She is also sure that Colin also got Ian killed to tie up loose ends. Ryan tells her that the partial print does not match to Colin’s prints.
Kate knows that Colin will take help from some trusted friend to do the killings for him. She tells Ryan and Esposito to do some digging. Kate gets a call from the Deputy Director. He tells her that she got the job. Later, Ryan tells Esposito about Colin’s half brother, Martin Tillage. He was one of the guests at the hotel Erika was staying at. They decide to bring in Martin. Kate goes to meet her father, Jim, for some advice. She tells him that she wants the job in DC. She also tells him that she is not sure about her relationship with Castle.
She doesn’t know where it is headed to because they never spoke about marriage. Jim tells her to tell Castle what she wants. Later, she arrives at the precinct to interrogate Martin. She tells him that his prints were found with Erika’s blood on it. Martin agrees to give a statement implicating Colin for the murders. Case Closed. Esposito and Ryan ask Kate what is going on. Kate tells them that there is something she needs to tell them, but someone else needs to be told first. Kate goes to meet Castle. Castle proposes her for marriage. She is at a loss for words. The season ends.