Castle

Opens with a body floating in a boat.
Castle is at home talking to Alexis and Martha. Martha is taking a class: introduction to life coaching. Alexis asks if Castle can chaperone a DC trip, Castle says he has to stay home and chaperone Martha so she doesn't get into mischief. Castle's phone rings with Bach's Toccata & Fugue, he answers the phone by saying who was murdered and was it gruesome.
At the park, Esposito says the victim is Donny, who was a senior at a private school. Dr. Parish says there is a single gunshot wound to the chest and he wasn't killed in the boat, he was moved. Beckett tells everyone to look for the place where he was shot.
Beckett and Castle talk to the victim's parents, asking them why he might have been at the park. They said he was probably with his tight group of friends. Castle asks if they moved into a smaller house recently, and if Donny was depressed because of their money issues.
Out in the hall, Castle wonders how they could still afford the private school. Beckett asks if he knows the school, and he says that he's been kicked out of all of New York's finest educational institutions at least once, but complains that now they all claim him as alumni and want money.
They then talk to the principal at the school, saying that Donny went onto a scholarship. He points them toward Donny's group of friends.
The friends say that they were with him the day before, but they all headed home around 5. They say they don't know why he was in the park, Beckett says she went to a private school too and she understands their life. They open up and say that Donny got into dealing drugs in the park because of his money problems. Castle later asks to see the photos of her in her private school uniform. Beckett says that she went to public school, she only told them that to make them feel comfortable.
Ryan shows Castle and Beckett the pool of blood by a bench in the park, and says that there was a 911 call last night about 50 yards away, and that the area is notorious for drug dealers. Castle surmises that the killer moved the body because he's a drug dealer and wanted to throw the cops off his scent by moving the body somewhere else.
At the station, the 911 caller is looking at mug shots, and points out a drug dealer.
Beckett talks to the drug dealer in interrogation, he says that he was in the park and he saw Donny last night, and that he sold to Donny and all of his little friends.
Beckett and Castle confront the friends because they lied about being with Donny in the park last night. They said they often go to the park and party, last night they were there and a guy came over with a gun asking Donny for the money he owes him - $200, and Donny didn't have it so the guy shot him. They say that they might be able to recognise the guy.
In a line-up, Romy points out the drug dealer.
At home, Castle watches Alexis sleep and notes how adorable she is. He asks her if she does drugs, she shoots him down with logic that she's clean. He makes sure that she knows she can tell him anything.
Castle comes rushing into the station asking why Donny's friends didn't give the drug dealer the money he wanted seen as they were all loaded. Castle says that the drug dealer is pointing to the friends and the friends are pointing to the drug dealer as the murderer. Esposito asks why the friends would point to the wrong guy, Castle says it's because one of them did it.
Beckett and Castle go to the drug dealer's cell, and get him to admit that Donny sold for him. He says that he heard the gunshot but wasn't there when it happened.
They go and talk to Romy and asked why none of Donny's friends spotted him the money the night he died. Romy confesses that Donny getting shot was an accident when they were playing Russian roulette with a gun, without bullets, but a bullet was in the gun and they didn't know it. She says that Max shot Donny.
They go and look for Max at his house, but his mother doesn't know where he is. While they're there, Beckett gets a call saying that Max committed suicide at the park.
Castle is a home, Alexis confesses that she once caught the subway without paying, but the next day she paid twice and didn't ride. Castle says that if that's the worst she's done, he a lucky man, and that as punishment she has to have ice-cream for breakfast.
At the station Beckett shows Castle ballistics that prove that Max used the same gun on himself that he used to shoot Donny. But they both wonder why he would have gone back and moved Donny's body if he was so distraught over killing him.
In autopsy, Dr. Parish says that it looks like suicide, but isn't. Max has abrasions on his index finger which show that someone helped him pull the trigger, and his blood alcohol level was .28, and he may not have even been conscious he was that drunk. Beckett concludes that Max was murdered. Beckett thinks that one of the other kids put the bullet in the gun while they were playing Russian roulette without Max knowing.
Beckett questioned the kid who owned the gun, asking where he was last night when Max killed himself. He said he was with Brandon.
Beckett and Castle spoke to Brandon at the school. He said they lied because they wanted to protect Max. When they asked where he was when Max killed himself, he quickly answers that he was with the kid who owned the gun. Beckett comments that all the kids always have their stories straight, and that's why she doesn't believe him. Brandon shows them a video file on his phone of Max shooting Donny.
In the car, Beckett notes that Brandon was smart to keep the video file, because otherwise they'd all be facing a murder charge. Castle wonders why they took a video of it at all, seen as it wasn't the first time they'd done it... what was so special about that night? The only reason to film it would be if you knew something was going to happen.
They find out that Donny and Amanda were dating, and Amanda was the one who made the video, so they bring her in. She tells them that Donny broke up with her a month ago because she cheated on him with Brandon. It turns out that Brandon told her to make the video that night.
They go and talk to Brandon and tell him that his alibi doesn't check out for the night that Max died. Brandon acts cocky, as if they can't prove anything, and that he has a video that proves that Max killed Donny.
Back at the station Ryan and Esposito say that no one remembers seeing Max or Brandon last night. Castle figures out that if Max and Brandon were together, their phones would have linked via Bluetooth, and Beckett realises that there would be a record if this had happened, and there was.
They interrogate Brandon and he accidentally confesses when Castle tricks him. Beckett tells Brandon that they probably wouldn't have figured it all out if Brandon hadn't gone back and moved Donny's body.
At home, Castle says he'll chaperone for Alexis, Martha tries to hurry them out the door, but she's too slow... her party guests have already arrived.