Someone has weakened the protective walls of Camp Half-Blood by poisoning the tree of the demigod Thalia, leaving the campers vulnerable to future monster attacks.
During the chariot races, Stymphalian birds attack, but Percy and Annabeth distract them with Chiron's boombox and Dean Martin's version of "Volare", enabling the Apollo campers to shoot them down.
As a reward for winning the chariot race, Tantalus sends Clarisse LaRue to find the Golden Fleece and forbids anyone else from leaving camp without his permission.
Aided by a hippocampus that Tyson names Rainbow, the trio reach the Princess Andromeda, a cruise ship filled with monsters and demigods that allied with the titan Kronos.
Realizing that she has turned dozens of men into guinea pigs, they escape on the pirate's ship Queen Anne's Revenge, which Percy can control, while leaving Blackbeard and his crew to deal with Circe.
Before journeying to the center of the island, Percy and Annabeth meet Grover, pretending to be a female cyclops to trick Polyphemus into not eating him.
They find the Golden Fleece, but Polyphemus destroys the Queen Anne's Revenge, forcing the group to escape on Rainbow and a few other hippocampi.
Taken aboard the Princess Andromeda, Percy makes contact with Camp Half-Blood through the goddess Iris and tricks Luke into confessing, exonerating Chiron, who is reinstated.
Having cleared the obstacle of Polyphemus luring in searchers through the Golden Fleece, Grover is given a furlough on his quest to find the god Pan.
The camp holds a second chariot race, which Percy and Annabeth win with the help of Tyson, who departs after accepting an offer from Poseidon to work in the god's underwater Cyclops forges.
[9][10][11] "In a feat worthy of his heroic subjects, Riordan crafts a sequel stronger than his compelling debut," said a Publishers Weekly review.
[12] School Library Journal singled out the novel's plot and main character, stating, "Percy is an appealing kid, and the subject of a chilling prophecy may resonate with readers.
"[10] Child magazine wrote, "Featuring the cliff-hangers and sassy attitude kids loved in The Lightning Thief – plus a surprising new family secret – this outstanding sequel should win over a fresh legion of fans.
"[13] Children's Literature called the writing extraordinary[13] and added, "This book, sequel to The Lightning Thief, is an amazing mixture of mythology and young adult fiction.
"[18] Following the box office and DVD success of the movie, Percy Jackson & the Olympians: The Lightning Thief, a film adaptation of The Sea of Monsters was first reported in early 2011.