About nine years ago, little Sean Kendrick watched as a horse ate his dad. Mr. Ed must have been really hungry that day, and they were all out of peanut butter.
Flash forward to the present. The island of Thisby is bustling with activity as the bloodthirsty water horses, known as capaill uisce, emerge from the seas dripping wet and hungry for human flesh. It's like the Galloping Dead, except these horses aren't zombies.
Puck Connolly, née Kate, has decided to enter the Scorpio Races—an annual event in which all the people who manage to catch and tame a water horse race them. Many people die. The Kentucky Derby this ain't. Puck decides to ride her own horse, Dove, in the race, half because she can't afford to purchase a water horse, half because no one will sell her one, and half because she's scared of them. Okay, that equals 150%, but you get the point.
The journey to the races is difficult, and not because Puck is fighting to keep her fingers from being bitten off. She has to face a lot of prejudice as a result of being the first female to ever try entering the races. She's the Sally Ride of water horse racing. To make matters worse, her brother Gabe has decided to leave Thisby, and if Puck doesn't win, the family will lose their house.
Meanwhile, Sean Kendrick—now all grown up—is a studly sea-horse whisperer, training his horse, Corr, for the races as he does every year. Tensions between Sean and the stable owner's son, Mutt, have reached a new high. Jealous of Sean's success, Mutt tries to sabotage Sean every step of the way—attempting to get him killed, trying to injure his horse, peeing in his boots, etc. The peeing in the boots part is just gross.
Eventually, Sean and Puck team up to train together for the races. Along the way, Puck tries to understand why Gabe is leaving, and she grows closer to her other brother, Finn, who sells his car so they can afford food for Dove (Puck's real horse who she'll be racing). No one would sell her a water horse, and she thinks that having a real horse gives her an advantage.
Sean has money problems of his own. He wants to buy Corr, but Mr. Malvern won't sell him. An American stable owner, George Holly, tries to hire Sean, but he won't leave without Corr. Finally Sean convinces Malvern to sell him Corr if he wins the races one more year.
On race day, Mutt Malvern makes his last move. Knowing Sean has the hots for Puck (they've been smooching ever since Sean went to Puck's for dinner), he gets his scary horse, Skata, to attack Dove. Sean intervenes, driving Malvern into the sea, never to be seen again. Puck wins the Scorpio Races and all the money that comes with it. Hooray!
She wants to help Sean, but she doesn't have enough money to pay off their house and buy Corr. Luckily, nice brother Finn ended up betting on Puck to win. She was a long shot, so he's rolling in dough, and he lends Puck the cash to buy Corr.
Sean buys Corr and decides to set him free, believing he'll be happiest at sea. Corr has other plans, though. He refuses to go to sea and returns to Sean. They all live together happily ever after.