Shawn, Gus and Henry return from a night at the movies to find that Henry's younger brother Jack is back in town and has broken into the house. Shawn finds him crouched by the coffee table in the living room. The guys are elated to see him but Henry is less than thrilled. Where they have always been enamored by Jack's wanderlust and fantastic stories of his worldwide adventures, Henry has always known his younger brother to be unreliable and irresponsible. Jack always wants something, and when Henry questions his motives for being back, he confesses that he wants their help to find the French pirate Bouchard's hidden treasure. Henry claims the legend of Bouchard's cache of stolen Spanish gold is just a story and the treasure doesn't exist. But Jack proves him wrong as he pulls out an ancient treasure map he discovered while traveling in Argentina. See, Bouchard, after a battle with the Spanish fleet and the Chumash Indians, made off with the gold then disappeared at sea, only to turn up in Argentina years later. Now Jack, armed with a map and a psychic detective, is positive he can find the treasure, and offers Shawn and Gus a fifty-fifty split. They jump on board.
The next morning, they meet Jack at a mission, where they impersonate professors and are able to gain access to the mission's private art collection. According to Jack's research, an important clue is hidden there somewhere and it has to do with the number 152. When Gus asks why they just don't use the map, Jack confesses that a part of it has been ripped away - the place that tells them where to start. But he knows with Shawn's psychic ability and the clues hidden in the mission, they will be able to figure out where to go. Shawn looks at the map and sees hidden in the words letters that spell out the French word for "painting." They find a painting of Bouchard in the collection that, when lined up with the map, points to a lighthouse as their first clue - a lighthouse with the historical designation number 152. But when they get to the lighthouse, they are immediately set upon by a group of armed men, who, it turns out, are Jack's original partners.
The men tie up Shawn, Gus and Jack in a nearby cabin. Jack tries to talk their way out of it by insisting that he only enlisted his psychic nephew's help with the search in exchange for a finder's fee, but the men are skeptical until Shawn is able to "psychically" discern personal information on them. Shawn then offers them the map and sends them in the wrong direction. The men leave the three of them tied up in the cabin and they are able to call Henry for help, who brings the three of them back to his ransacked house. Jack is upset with Shawn for giving away the map, but Shawn says he doesn't need it anymore. He has it all in his head. He also knows that the men didn't trash the house looking for the map. He remembers seeing Jack by the coffee table and reaches under it to produce the old key Jack hid there. That is what his old partners were looking for. Jack said he found it in Argentina as well, but it was useless without the completed map. Now that they have both, they are back on track to find the treasure. This time Henry insists on coming with them to make sure they are safe and keep them from doing anything illegal.
Shawn is then able string together the clues from the past and figure out where the treasure is hidden, but Jack's old partners show up again and chase them right into a hovering armed helicopter. Jack and his partners disappear quickly, and Shawn, Henry and Gus are taken into custody. Back at the police station, Lassiter and Juliet tell them that the men in the helicopter are from the Spanish government and are in charge of recovering Spanish antiquities. Their boss, Andres, tells Shawn that they have also been looking for the treasure but have not been having any luck. They are requisitioning Shawn's help to find the gold and return it to Spain. Shawn recalls the map again from memory and tells them they are looking for painted caves. They figure out that means the painted caves of the Chumash Indians. Andres takes Shawn and Gus with him, strong arming Vick, Juliet and Lassiter to stay behind, insisting this is out of their jurisdiction.
Once in the cave, Shawn and Gus dig to discover the treasure, but just as they pull it out of the ground, Shawn turns on Andres and his men and runs out of the cave with Gus and the gold. As they run through the woods, Jack's old partners catch back up with them and join the chase. When they reach a clearing, Jack pulls up in a car, but instead of rescuing them, he takes the gold and tells Shawn he'll meet back up with him at the Red Robin. He takes off, leaving them stranded as Andres, his men and Jack's partners all come out of the woods guns trained on the guys. Just as everyone is discovering they've been double crossed by Jack, the police arrive and arrest all of Jack's associates. Shawn explains that he figured out Andres and his men weren't Spanish, they were Argentinean, and so mostly likely another set of Jack's partners. He slipped Juliet a note on their way out of the station, warning her Andres was a fake and telling her to follow them. With all of Jack's partners in custody, Shawn goes to meet him at the rendezvous, but Henry shows up instead. Shawn tells Henry he realized he was right about Jack all along, and switched out the gold before he gave Jack the bag. Gus has delivered most of it safely to a museum, leaving Jack on the road with a bag full of rocks, frustrated but still proud of his quick, creative nephew.