The barrier that protects the half-breed camp from the outside world--and enemies--is weakening because it was poisoned by Luke, the half-human son of Hermes. This barrier gets its power from a huge pine tree that used to be the daughter of Zeus. So Percy, Annabeth, the satyr Grover and Tyson, the cyclops half-brother of Percy, go on a quest to retrieve the Golden Fleece, which has the power to cure the tree. But it's on Circe's island, in the middle of the Sea of Monsters, otherwise known as the Bermuda Triangle. Not only do they have to contend with Luke and his followers, who want to resurrect Chronos, leader of the Titans, but the Cyclops Polyphemus, who guards the Fleece.
I enjoyed this as much as the first movie, though this series is basically the Harry Potter of Greek Mythology. There's an amusing scene with Nathan Fillion (Firefly) as Hermes and some very cool effects, particularly an aquatic horse and the giant sea monster, Charybdis, which swallows them like Monstro the whale. Recommended if you enjoyed the first movie.