We pick up where we left off at the end of last season -- Shawn has just realized his father's life is in danger. He rushes to find him, but it's too late. One of Henry's old partners, Jerry Carp, has already shot Henry because Henry found out Jerry was involved in a police scandal years ago, and Jerry needed to keep Henry quiet. Shawn rushes Henry to the hospital. After Henry makes it out of surgery okay, Juliet urges Shawn to not get involved with catching Henry's shooter. Shawn doesn't listen; he recruits Gus and they set out to find and arrest Carp.
Shawn and Gus head to Carp's beach house to investigate. There, Shawn sees that Carp's home is far too nice for a retired cop. He determines that Carp is involved in a much larger scandal, and someone is helping him flee the country to avoid arrest. Just then, Lassiter and Juliet show up and are annoyed that Shawn and Gus got there first. In Carp's basement, they find a storage filled with automatic weapons and ammo. Shawn realizes most of this loot came from a place called "Red Chief Firing Range."
Shawn and Gus head to the firing range to investigate. Shawn asks to see the owner, but the employees say the owner, Jack Atwater, doesn't come out unless there's trouble. Right then, Shawn remembers that Jack Atwater was also one of Henry's old partners -- the guy the SBPD put behind bars at the end of last season. Shawn and Gus go to visit Atwater in prison, but he is shot by a stranger from far-range before they can get any answers.
Back at the SBPD, the gang searches for leads on Atwater's shooter. Vick has a private conversation with Shawn. She more or less lets him know, without actually telling him, to go after Carp. Now armed with the Chief's approval to keep investigating, Shawn goes undercover as "Soupcan Sam" and heads back to the firing range to get a look at their "Hall of Fame" list -- a list depicting the best shooters that ever shot a gun at Red Chief. Shawn believes that whoever took that perfect long-range shot at Atwater, must be on this list. Shawn notices one man's name, a J.E. Vest, and realizes he saw this guy in a photo with Carp. Vest must have been involved in the scandal. He could also be the shooter.
Shawn and Gus head back to the hospital to ask Henry about J.E. Vest. Henry tells him that Vest now works Private Security at a non-profit organization called "Feed Everyone." Shawn realizes that the owner of the organization, Julian Drake, is at a bookstore doing a signing. Shawn figures Vest is probably there working security for him.
At the bookstore, Shawn and Gus see Vest and immediately approach him. Shawn and Gus end up causing a scene. Vest takes control of the situation and throws the guys out of the bookstore. Vest steals one of Shawn's business cards, and now he knows who he is and where he and Gus work. At this point, Gus wants to back out, but an eager Shawn convinces him to keep going.
Back at the Psych office, Shawn and Gus sit down to discuss their next move. But before they can get a word out, their lives are threatened by a landmine that has been planted under their couch -- a special landmine that will trigger when they stand up. Shawn and Gus realize that Carp had a cache of these same landmines in the weapons closet at his beach house. Shawn and Gus manage to call the SBPD to come in and freeze the device before it blows. The SBPD are able to disarm the bomb and Shawn and Gus are saved.
Shawn and Gus' next move is to find Vest, so they head to the headquarters of "Feed Everyone." There, Shawn realizes that "Feed Everyone" is a scam organization and that Drake's "non-profit" is supplying weapons to Africa, not food. Shawn also realizes that Carp's been the one supplying Drake's scam with automatic weapons, which explains why Carp was living in a swanky beach house. Just then, Drake's other security guard shows up and points a gun at Shawn and Gus. But it turns out he's FBI and has been investigating Drake undercover for months.
Back at the SBPD, Vick informs everyone that they have to back off the Carp case because the FBI now has jurisdiction. But Shawn knows he needs to catch Carp himself -- for his father. Shawn follows a lead he saw in the FBI officer's case file to a house.
At the house, through binoculars, Shawn sees Carp there and decides that he and Gus need to break in immediately and bring Carp down. Against Gus' wishes, Shawn tries to break in, but the house is fortified and the situation proves to be too dangerous. They also nearly destroy the blueberry in the process.
Later, Shawn and Gus are surprised to learn that Lassiter is determined to stay on the case despite the Chief's order to stay off of it. Lassiter knows they are too close to finding Carp to stop now. So Shawn and Lassiter devise a plan that involves sending an order of Chinese food laced with a vomit-inducing agent to the house where Carp is at.
Later that night, at the house where Carp is at, Shawn and Lassiter see one of the guards start to vomit; they now know their plan is working. With the guards now distracted by their sickness, Shawn kills the power in the house and makes his way inside.
He searches throughout the house and finds Carp. Shawn is finally face to face with his father's shooter. Shawn pulls a gun on Carp and starts to lead him out of the house for arrest; but Shawn is stopped when Julian Drake shows up with a gun pointed at Shawn's back. Shawn is able to kick Julian off of him, but in the mess of the action, Carp manages to pick up a gun and point it at an unarmed Shawn. Just before Carp is about to pull the trigger, Juliet shows up and takes him down, saving Shawn's life. It's finally over.
Back at the hospital, the doctor tells Shawn that Henry is on the road to recovery and is going to be just fine.