Davey is a fourteen-year-old boy, and he is riding a carousel with his cousin Frank and their friend Riley. Davey wants to keep the blue gem on his horse Lysander. Frank takes a pocket knife and removes the gem, giving it to Davey. As they are walking home, Riley asks what they were doing on the carousel. Davey shows him the blue gem. Riley takes it and throws it across train tracks that are down a hill. Frank gets angry, and punches Riley. Riley falls down an embankment, onto a pipe that goes through his leg.
Twenty-five years later, Davey is a free-lance heritage analyst, so he knows that the carousel was made by a man named Charles Carmel. Davey is sitting at a bar in Ireland, after winning a free trip there. He sees a woman singing who interests him. All of a sudden, Frank walks in. The two cousins have not seen each other in ten years. They start catching up, and realize they won the same exact prize to visit Ireland. They also have endured similar tragedies. Davey lost his wife five years prior in a car accident; Frank's wife had left him a year prior after losing their six-year-old son in a house fire. They also suffered similar career setbacks, Davey's due to an elaborately staged hoax at a museum and Frank's from a misdiagnosed cancer. 
The two cousins are very confused about what is happening, when their waitress hands them a business card, saying a man upstairs wants to see them. The cousins go upstairs to the room, where they see a man with a walking stick and an expensive suit. They soon recognize the man as Riley. Riley says that they must complete an old incantation. He says that the man who commissioned the carousel they rode as children was a stage magician named Chinder, and his favorite poem had a line that said "it was an Ancient Mariner, And he stoppeth one of three." Chinder had this line engraved on a a carousel named Mariner's Round that Riley has located.
Riley reveals that the mounts on this carousel are meant to fulfill one's wishes, and the wood is from shipwrecks. Riley takes out the jewel from his pocket, revealing that he pocketed it and never threw it. He asks Davey what the name of the horse it came from was, then is astonished when Davey says Lysander. Riley explains that Lysander is the name of a seventeenth century ship that went missing, and the ship is part of what got him interested in auctions and maritime archeology. Riley convinces Frank and Davey to come see the Commission carousel, for old time's sake. They agree.
The carousel is in a warehouse. Riley wants them to ride the carousel, and reveals that the jewel Davey and Frank stole was never meant to go on their hometown carousel, it was meant for this carousel. He then reveals that he has been "tinkering" with their lives as revenge for the accident, and he caused all of the tragedies. Riley threatens to kill Frank and Davey if they do not ride the carousel and restore the jewel to its function of fulfilling the heart's desire. Frank and Davey reluctantly get on. 
As they ride the carousel, Davey suddenly ends up at the bar he was at earlier, with the woman singer who he was interested in. Everyone is signing an old sea-chanty as a round, just like how a carousel goes around endlessly. Davey laughs and joins the singing. Frank ends up at an old farmhouse, where many people are celebrating and dancing around. Frank's ex-wife is there, and he is happy even though his son is not there. Riley ends up on the deck of the Lysander, the 17th century ship that disappeared. The crew are happy to see that Riley has the gem, and they are able to finally end their struggle and descend into the water.