The Sea of Monsters is the second installment in the series, released on April 1, 2006.
Camp Half-Blood is under attack when Thalia's tree, which guards the borders of the camp, is poisoned and slowly begins to die. In order to save the tree and the camp someone must recover the Golden Fleece, which is somewhere in the Sea of Monsters. At the same time, Percy finds out that Grover, who has left on a quest to find the missing god Pan, has been caught by the cyclops Polyphemus and that the Fleece is on Polyphemus' island. Together with Annabeth and his half-brother Tyson, a cyclops, Percy sets out to rescue Grover. Meanwhile, Clarisse La Rue, daughter of Ares, is sent on an official quest by Camp Half-Blood to retrieve the Fleece. The trip to the Sea of Monsters is long and hazardous and along the way the heroes encounter several dangers including Scylla and Charybdis, the sorceress Circe, the Sirens and their former friend Luke Castellan. Percy also learns about a prophecy from the Oracle about a child of one of the three most important gods (Zeus, Poseidon and Hades), which in all likelihood refers to Percy, playing a vital part in the success or failure of the resurrection of Kronos the Titan-King. The heroes eventually retrieve the Fleece and restore Thalia's tree but also unknowingly revive Thalia herself, daughter of Zeus, who had been turned into the tree by her father when she died.
The book was adapted into a film by Thor Freudenthal and 20th Century Fox, under the title Percy Jackson: Sea of Monsters, and was released on August 7, 2013.