"Percy Jackson and the Sea of Monsters" is the second book in Rick Riordan's "Percy Jackson & the Olympians" series. The story follows the adventures of Percy Jackson, a demigod son of Poseidon, as he embarks on a quest to save Camp Half-Blood, the sanctuary for demigods.

The story begins with Percy having a relatively peaceful year at school until a game of dodgeball turns into an attack by Laistrygonian giants. Annabeth Chase, Percy's friend and fellow demigod, arrives to help, and they escape with Percy's half-brother, Tyson, who is revealed to be a Cyclops. They make their way to Camp Half-Blood, only to find that the magical borders protecting the camp are failing. Thalia's tree, which generates the protective barrier, has been poisoned, leaving the camp vulnerable to monster attacks.

Chiron, the camp activities director, is unjustly accused and replaced by Tantalus, a disgraced mortal from Greek mythology. To save the camp, Percy learns that the Golden Fleece, a magical artifact with immense healing powers, can heal Thalia's tree. However, the quest to find the Fleece is assigned to Clarisse La Rue, daughter of Ares, much to Percy's dismay.

Despite not being officially chosen for the quest, Percy decides to set out to find the Fleece with Annabeth and Tyson. Their journey takes them to the Sea of Monsters, known to mortals as the Bermuda Triangle. Along the way, they face numerous challenges, including an encounter with the sorceress Circe, who temporarily transforms Percy into a guinea pig, and an attack by the hydra.

They eventually find Clarisse, who has been captured by the Cyclops Polyphemus. Polyphemus guards the Golden Fleece and has imprisoned Grover, Percy's satyr friend, who was previously searching for the Fleece. Percy and his friends rescue Grover and Clarisse, outwit Polyphemus, and secure the Fleece.

Returning to Camp Half-Blood, Clarisse is hailed as a hero for bringing back the Fleece. The Fleece is placed on Thalia's tree, and the tree is miraculously healed, restoring the camp's protective barrier. However, the Fleece's power is so strong that it revives Thalia, Zeus's daughter, who had been turned into the tree years before. Thalia's return introduces a new dynamic and potential complications, as she is another possible candidate for the Great Prophecy, which foretells that a child of the Big Three gods (Zeus, Poseidon, or Hades) will either save or destroy Olympus.

The book ends on this cliffhanger, setting the stage for the next adventure in the series.