Attorney General Maxwell's son Dylan is getting married. Only problem, the engagement ring, a family heirloom worth $5 million, has been stolen from the safe at the hotel where the wedding is to take place. Shawn, at the police station on another case, sneaks into the interview room where Dylan and his fiance Bethany are waiting, along with Dylan's sister, and Bethany's maid of honor, Lacey. In the minute he has before Lassiter comes in and catches him, he's able to get all the information on the case -- along with an invitation to the wedding that weekend.
At the hotel, Shawn is able to listen in on Lassiter's briefing on the case. After the rehearsal dinner, members of the wedding party, including a drunk and stumbling best man, enter the safe room with the facilities manager, Dietrich Manheim, and place the ring inside the vault. Sensors indicate that neither the box nor the vault, which was on a time lock, was opened until the following morning. A video of the vault room confirms this. Since then, Dietrich Manheim has not shown up for work, and a review of his computer searches indicate that he was looking into buying a boat and making other major purchases.
By convincing a psychic-skeptical desk clerk that he is an Interpol agent working independently of the American police, Shawn is able to get unrestricted access to the front desk computers for a short time. He uses the opportunity to actually make keys for the rooms of all the suspects. With the keys, Shawn eliminates nearly all of the suspects and stumbles upon the wedding party where he finds out that Lacey has a wild side, the best man has been missing since the ring disappeared and that same night, the florist quit and everyone had to get together to hand make the bouquets. Actually, that's why Dylan figures his best man is missing -- he's lazy and didn't want to help with the flowers.
With their final key, they sneak into Lassiter's hotel room and go through the case files. They find out that the Attorney General has an insurance policy on the ring that will pay out only as long as it belongs to him -- which is until the wedding takes place.
Gus decides they need to get a look at the safe. Since the vault is still a crime scene, Shawn says the closest they can get is the airshaft above it. When they open the dumb waiter, which leads to the shaft system, they find the body of Dietrich Manheim, Lassiter's chief suspect. After Shawn "psychically" finds the body for the police, keeping them busy, he and Gus break back into Lassiter's room and review the vault footage again. Oh, and also host the bachelor party. Not just a fun time, but also a great way to get information out of drunken suspects. Unfortunately, the only thing they find out is that Dylan never wanted the ring in the first place. Shawn asks him about his father's finances, but he passes out before they get an answer.
Shawn doesn't get any farther with Lacey, either. Before she can answer his question about her dad's finances, she's distracted when they arrive at the cooler with the bouquets locked inside. She points hers out and, being the compulsive that she is, asks Shawn to break in so she can make hers better. He can't.
The day of the wedding, Shawn shows up in the bride's suite as everyone is getting ready. The maid arrives and starts to pass out the bouquets when Lacey freaks out. She needs to have her specific bouquet. She planned the wedding and has been completely uptight about everything. Shawn calms her down and she composes herself enough to perform a magic trick with her fingers for the nervous flower girl.
Shawn knows what happened! Unfortunately, he figures out the final piece in the middle of the ceremony. His only choice is to stop the wedding and reveal all in a huge psychic episode. Lacey was supposed to inherit the ring. She counted on it as her inheritance. She would have taken it sooner, but since sometimes engagements end, her father decided to keep the ring locked up until the wedding. So the only time Lacey could get it was at the rehearsal -- where she insisted it be used. The ring never went into the vault. She tripped the best man and then pretended to straighten the ring in the box, using her quick hands to palm it. Everyone was fooled except Manheim, who threatened to blackmail her. Their argument got rough, and he ended up dead. She would have gotten away with it but when the call came about the florist, she had no time to hide the ring, and so was forced to wrap it into her bouquet, which was locked up until the ceremony.