In 1986, Henry is teaching a young Gus and Shawn, in scout uniforms, about the third law of motion: For every action there is an equal and opposite reaction. Shawn says that it applies to the nature of man because when you push they push back, because man is a stupid creature who would rather fight than use his brain. Henry says that losing is unacceptable because losing is for lowlifes and quitters. Henry is going to set off a rocket and one of the boys will have to retrieve it for a hot fudge sundae. Only the winner gets a sundae not the loser. Henry sends of the rocket. Shawn and Gus ran after the rocket. Gus finds a parachute. They both run back and they find that Henry has already retrieved the rocket, because he took a shortcut. Present day, Shawn and Gus are watching the annual Civil War reenactors practicing. When Gus points out that the event doesn't happen for another week, Shawn says that the practice is the best part. Lassiter is one of the re-enactors. Gus notices an ugly beard and mustache that Lassiter wears for the reenactment and wonders what "died on [Lassiter's] face". The re-enactors start the battle. Lassiter gets off his horse and starts to critique the soldiers. Lassiter feels that the battle they are reenacting is special because his great great-grandfather, Muskum T. Lassiter. Shawn wonders what a Muskum is, and Gus tells him it was a common name of the time. Shawn gives him a weird look, and Gus says he's been watching the History channel. Shawn tells Gus to start watching ESPN. Juliet arrives. Lassiter yells at Sally Reynolds for missing her cue. She says she was stuck in the medic tent because someone had a question about their insurance policy. Lassiter takes himself very seriously and when Sally tells him there is no reason to "get Nasty", Shawn yells out his agreement. He is surprised that Lassiter hears what he says. Lassiter continues to berate Sally as he makes his way over to Nelson Poe, the man who was shot off his horse, and he realizes that Poe was really shot and is dead.
Shawn tries to convince Lassiter to let him onto the case. He tries to say he was drawn to the case but Lassiter knows he was there every time they rehearsed. Lassiter tells him to get lost, and Shawn sees into the evidence room and sees people examining the bullet holes on Nelson's jacket. Shawn fakes a vision as Lassiter walks by he says he sees the bullet going straight through his heart, and that it was not an accident. Lassiter is angry that Shawn knows it went through Nelson's heart. Lassiter says the case is personal because it was one of his soldiers killed on his watch and he wants to be the one to clean it up. Shawn mocks Lassiter's choice of the words "we clear?" to make sure that he and Shawn are on the same page. Lassiter leaves to go into the police room.
Shawn sneaks into the room. Vicks is explaining to the reenactment committee that the event is still going to happen, pending the investigation. Vicks says that if they find any evidence that it wasn't an accident, and they can't make an arrest, they will reevaluate before the event. Vicks hands it over to Lassiter, who is still wearing his beard from earlier. Vicks makes him rip it off his face, everyone flinches. Lassiter feels that there are no indications of foul play. There is some chemistry between Sally and Shawn in the back of the room. Lassiter says that a live round probably got mixed in with the blanks, and this is supported by the munitions expert, Griffin Mahoney. Griffin says that they had live round target practice the past weekend, and that a musket ball must have gotten stuck in one of the rifles. Mahoney says that it was a terrible accident. Sally and Shawn continue to look at each other but look away when the other one notices in the back of the room and Juliet comes into the room. Lassiter tells them it is an official police investigation, and hands the presentation over to O'hara. Juliet has a model of the battlefield. She points out that Nelson was probably shot of his horse, and rolled into the creek bed. Lassiter says because they knew the direction he was facing they know where to focus their investigation. Shawn picks this moment to come form the back, saying that 3 is the bullet is missing. He gets Lassiter to tell the Chief he had an episode and that is how he knows the information. Shawn picks up the model that represents Nelson, and says he can hear it saying that he was shot through the heart and it was not an accident, he was murdered. The Chief hires Shawn to investigate. Gus and Shawn are walking across the battlefield. Gus is mad because it doesn't seem like Shawn will be able to prove murder based on two bullet holes. Shawn says he checked up on Nelson and that no one ever liked him. Shawn doesn't have any more information than that. Shawn and Gus arrive to see Lassiter giving directions to his police force. Shawn sees a list of suspects. Shawn starts laughing and it annoys Lassiter. Lassiter tells Shawn that reenactment is a sacred trust. Gus admits that he is a bit of a Civil War buff and Shawn says he is just reading everything looking for information. Lassiter says that they are 95% sure that it was an accident, and if they are wrong Shawn and Gus will be invited to a press conference where they can say "I told you so". Back at the office, Shawn tells Gus about the 3 names checked off on the suspect list. Shawn then sets about using toys and the air hockey table to recreate the model of the battlefield. Gus is amazed. Shawn notices that only one person is close enough to have fired accurately at Nelson. It was George Cheslow, a person Shawn and Gus knew in 6th Grade. Gus says he is dentist now. Shawn is pleased with this news.
Shawn and Gus are waiting in George's office. The woman at the desk calls for "Mr. Guster", and Gus is angry at Shawn for setting up an appointment. Gus in the chair and he is chewing Shawn out even though he has a spacer in his mouth. Shawn understands what he is saying even though the dentist doesn't. The dentist asks Shawn why he has to be in the room, and Shawn tells him that Gus has a huge fear of being alone in the dentist chair. George's wife shows up, she is extremely hot. Shawn listens in on their conversation and watches her take George's money. Shawn asks how marriage life is treating George. George tells Shawn that his wife was having an affair with Nelson. He says everyone was past it. While George is looking for some gauze, Shawn notices that George is blind without his glasses. George tells Shawn that Lassiter won't let him wear his glasses because they aren't from the right period. Shawn and Gus realize that George is innocent. George finds a small cavity and he injects Gus with some anesthetic to Gus' objections that his teeth are perfect which fade to babble as he loses control of his mouth. Lassiter comes in and arrests George based on finding some letters at Nelson's that indicated the affair and that George threatened to kill him. Gus tells Lassiter that he has the wrong man and Shawn has to translate.
Shawn tells the Chief and Lassiter that he knew it wasn't an accident from the beginning and that George is innocent. Lassiter says that they have motive, means and opportunity. Shawn says that the exit wound was too small for the person to be as far away as George was. Lassiter makes Shawn leave because he has no other suspect. Vicks kicks Shawn out because "she's afraid he's right this time". Shawn leaves saying he will get Vicks a name and Lassiter a woman. Henry is at Mahoney's shop ordering a gold pocket watch for Shawn. Henry has a problem with making inscription to lovey-dovey. He originally has the message include his last name, but Mahoney tells him this contradicts the statement. In order to avoid spending too much money, he cuts the inscription from "Shawn, don't lose this watch, Henry" to "Don't Lose, Henry". Shawn shows up at Gus' office in Civil War uniform. He introduces himself as Lieutenant Crunch but then he has been promoted to Captain Crunch. Shawn tries to get Gus to join the regiment. Gus reminds Shawn that he is black, so he wouldn't fit in with the hardcore reenactments. Shawn tells Gus that he was think Gus could be like Denzel Washington in Glory. Shawn says that Mahoney will be making Gus something spectacular.
At Mahoney's shop, Mahoney informs Shawn that he got back so early that the uniform may not be ready yet. He says that Gus will be getting the best uniform they have. Mahoney takes them to the back of his store, where he keeps all of his Civil War memorabilia and props. Sally Reynolds is in the back. Shawn flirts with her, she flirts back. Sally wrote insurance policies for most of the men in the regiment. Sally even wrote Mahoney's insurance policy. Gus sees a gun and is able to identify it, while handling the gun he gets some stuff on his hands. Shawn finds some Brass buttons soaking in urine. Gus puts on his uniform, and he is not pleased with what it looks like.
Gus and Sally are on the battle field and Gus wants to know if there are any other uniforms he could wear. Sally says no but she thinks he looks good; she always liked the marching band. Shawn is playing video games, and sitting in a blow up chair. Shawn and Sally take a walk and they discuss the historical nature of the battle. Shawn asks Sally what she wants, but she mocks him by saying he should already know. She doesn't believe in psychics. Shawn and Sally kiss. Shawn loses his footing on the bridge and rolls down the hill. On the ground he notices a hole in the tree nearby. Gus comes up and Shawn tells him he knows how Nelson was killed.
Shawn shows up at his dad's door still in uniform. He tells him he lost his pocket watch. Back on the battle field Shawn is using a metal detector on the ground. Henry asks how Shawn could lose a watch with the inscription "don't lose". Shawn informs him that his birthday was 4 months ago not yesterday when he received the watch. Henry tells Shawn he didn't smile at him until 4 months so that's when the clock started for birthdays. Shawn finds a button in the grass. Henry offers up his opinion that killing your wife's lover is something that is rarely planned, even if it is planned, he isn't going to wait around for the cops to find the love letters. Shawn tells his dad that he figured out from where Poe was shot last night. Shawn then locates the bullet in the grass, and marks it with a pencil. Shawn pulls out his pocket watch. He had to trick his dad into letting him use his metal detector. Gus brings Lassiter and O'hara out to the battlefield. Sean is there in a "trance". He says he sees yellow, and a number 2, and graphite. Shawn leads Lassiter to the pencil using the hotter colder method from elementary school. Lassiter and O'hara are interrupted during Shawn's show, and leave. Shawn decides to run through the things they know from the killer's point of view. He climbs into the tree. Shawn figures out that Nelson wasn't the original target. Gus tells Shawn to look for a notch. Shawn finds the notch and it points to the hospital tent. Sally Reynolds was the original target.
Shawn and Gus steal Sally Reynolds' dress, so that Shawn can put on the dress to find out who the culprit was. They tell Juliet about the plan and she decides to be the one to take Sally Reynolds' place. Shawn and Gus help Juliet put the dress on over a bullet proof vest. They have to push her out of the tent. Gus and Shawn go to where the killer should have been but there isn't anyone there. Shawn and Gus find a tunnel under the tree. The tunnel goes to the other side. Shawn and Gus find Mahoney's tunic and boots in his tent by the tunnel. They realize that Mahoney did it but they can't figure out why. Shawn and Gus solve the case at the same time after hearing a C4 explosion.
Shawn and Gus hurry to go catch Mahoney in the act. Mahoney tries to scare Shawn and Gus away with a hand gun. As they back into the lobby of his store, there are a bunch of the re-enactors standing with their rifles pointed. Lassiter comes in and takes Mahoney's gun away.
Back at the police station, Gus is swinging a watch in front of Shawn to help him into a trance to go over the details of the crime. Mahoney blew up his own safe. He was going to rob himself and use the battle as a cover. Sally was in on the scheme the whole time, she wrote Mahoney's insurance policy. As Lassiter leaves Shawn and Gus act very friendly towards him say they are part of a team.
Back in 1986 Shawn and Gus walk through a field. Shawn asks Gus if he'll ever be as cool as him. Shawn feels uncomfortable around his father because his dad has never said "I love you". Shawn and Gus run into a tunnel yelling "CHARGE".