The scene opens with two police officers on horseback riding through the park. They approach a dead body. At the house, Castle makes a new thing for breakfast, a smorelet, a graham cracker, marshmallows and a chocolate bar wrapped inside of an egg. However, Alexis is grossed out and leaves out the door. Castle wants to know why she left in a hurry and Martha tells him that Alexis is in love. Castle is floored by the news and thought that he was Alexis's go-to-guy. Castle gets a phone call from Beckett alerting him another murder. Castle arrives on scene where the body from the beginning and he is only left in his underwear. Castle jokes and tells that he has the same type of boxers. Esposito and Ryan come up and tell that the body belongs to Daniel Goldstein. He has a downtown address and a downtown job, but the murder scene is not downtown. Beckett gets up to leave and Castle jokes again and asks if it is a thong for her. She ignores him.
Back at the station, Daniel's sister, Rachel, is there and she tells that Daniel was a math genius brother and a bit of a dork, worked on Wall Street at an accounting firm. Ryan tells that they found a print on Daniel's wallet. They meet with a felon named Roland D'Andre. He is not talking but Beckett wants to know why a gun matching the bullet hole in Daniel matches the 45mm that was found in his apartment and why he through her detective through a wall. Ryan tells Esposito that a nice turtleneck will cover the neck brace. Esposito wants D'Andre to fry and she tells that he will as soon as Lannie verifies that the bullet is a 45mm, but the unregistered gun that D'Andre has is not the murder weapon. She tells that the bullet was 200-years-old. Castle's brain starts moving and says that it is possible that Daniel is was killed by a time-traveling killer. Castle talks about his theory and tells that he needs the time machine. Ryan tells that it couldn't be a time traveling killer. The bullet would have been brand new. Castle tells that time travel causes rust. Abe Sandwich, ballistics expert comes in and tells that Antique guns can be fired. Beckett tells that they still can't find the clothes and tells Esposito that D'Andre is not the killer. However, Captain Montgomery comes over and tells that he will get the max for assaulting an officer. Castle suggests that the killer showed up naked and needed the clothes, like the Terminator. Beckett tells that she and Castle are going to Daniel's work and Esposito and Ryan are going to search.
Beckett doesn't know why someone would steal close and Castle wonders why Alexis has not told him yet to tell that she is in love with someone. Beckett tells Castle to leave it alone or Alexis will date someone horrible. They arrive at Berman Rose, the place where Goldstein worked, and meet with Adam Murphy. He says that the Daniel checked out around 9:30 PM. Daniel's assistant, Julia Foster, tells that everything was going fine that day except for phone calls from angry clients. They'd tell that Daniel created a financial product that many people invested and it tanked. Many people of their money and many were upset. Meanwhile, Ryan and Esposito are looking at web sites of antique gun collectors and the different investors and one individual to sticks out. Ivan "Yosemite Sam" Padowski, a gun collector and investor is on their list now and Beckett and Castle look at him and agree that he looks like Yosemite Sam. They meet with him and find out that he left a threatening message but claims that he would never do that. Beckett tells that they're going to run Ballistics on his collection of antique guns.
Esposito comes up to Beckett and tells that they got a hit on the parking ticket and that Daniel's car is a Delorean. Castle rehashes his time traveler theory. Ryan and Esposito check out the Delorean and see that Daniel had all the fixings to the original. A homeless man pushes a cart and Ryan sees that he has Daniel's clothing that were on him. It is a Victorian-style shirt with a blood stain on the chest. The man says that he found them in the trash and tries to fight off them. He rams an already hurt Esposito and Ryan takes him down and arrests him. He asks if Esposito is alright and he says that he is. At the labs, gunshot residue is found on the rest of Goldstein's clothing and it suggests that he also had a weapon that he fired. Castle is starting to believe that Daniel had a different life. At the park, they find another round bullet and Castle determines that they were 40 paces away and had an old-fashioned duel because of the clothing that he was wearing and that they stood and fired at each other at the stroke of midnight and had a duel.
Castle has an old fashioned gun and returns home with it. He is going to scare Alexis for fun, but instead finds Ashley, her friend, is actually Alexis's boyfriend. They are caught kissing and they freak out at the gun. Castle tells that he is not going to do anything and Alexis is upset that he barged in as he did. Alexis tells that she will call Ashley later and he tells Castle that he respects Alexis. Castle likes Ashley. The next day at the station, Castle tells of his close encounter with Alexis's new boyfriend Ashley and Beckett tells that they have no leads now. Ryan comes in and tells that Daniel met with Troy Kenworth's, son of Charles Kenworth and that Castle thinks that Troy wanted to get revenge. However, when Beckett interviews him, Troy tells that he was thankful to Daniel for getting him a job with a secret steam-punk society. They go there and they get in. Castle tells that steam-punk is part of a subculture which embraces the simple things of the past and mixes in things from the future. They talk to Mr. Peterson they say that they were using antique guns and Peterson tells that they fired the dueling pistols. Beckett verifies that they were fired resently and then sees Adam run away at the party and they chase after him. Ryan and a newly rested Esposito arrive at the party and open the door. Esposito is tackled by Adam.
Adam tells that he was in a duel with Daniel over Julia, Daniel's assistant. They wanted to make Julia like one of them and so they staged a duel. He tells that Daniel had the numbers calculated and there was no way that the bullets her going to reach them. Beckett and Castle kind of believe Adam's manslaughter plea and go and test fire the antique guns for accuracy. They cannot hit the targets and agree that it was manslaughter. Afterward, Castle is in futuristic gear and Alexis walks up to him and asks what it feels like when you are in love. However, Alexis keeps answering her own question and Castle doesn't get in a word. She tells Castle that she loves him and leaves. Immediately afterward, Beckett shows up and she tells that the bullet that hit Daniel, didn't match any of them. There was a third shooter. At the park, they find Adam's lead ball and Ryan tells that he did kill a squirrel. Beckett judges where the third shooter would have been in the park and they find clothing fibers on the tree. They know who the killer is and the murder weapon.
They go to Steam-Punk Society and talk with Troy Kenworth. He is taken into custody and confesses to the crime that he got tired of looking at Daniel's smug face. Castle and Beckett tell that the case is closed and Captain Montgomery tells if they heard about what happened to Esposito. Ryan wheels Esposito in and he is in a full upper back brace and everything, but it is a trick and Esposito tells that he is fine. A man named Josh comes in and Beckett's knows him and they are acting friendly. Castle drills him and asks about him and Beckett. They walk off arm and arm and all men are puzzled. Later on, Alexis tells that she is thankful for his help and she tells that they need to go a date. She tells that he will always be her Go-To-Guy and Castle realizes that it is a pity date, but he will take it.