Thus Percy Jackson and his companions, Grover Underwood and Annabeth Chase, a daughter of Athena, start on a journey to the underworld, facing numerous mythological monsters on the way.

Percy also learns about a prophecy from the Oracle about a child of one of the three most important gods (Zeus, Poseidon and Hades), 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.

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.

Thalia replaces Zoe, who dies, as Artemis' lieutenant.

Thalia's induction as a lieutenant of Artemis ensures that she will become immortal, never aging to 16, thus escaping the Great Prophecy and leaving Percy to fulfill it.

At the same time, Percy finds out that Grover, who has left on a quest to find the missing god Pan, has been captured 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.

When Riordan ran out of myths, his son suggested that he make up new stories using existing mythological characters and new ones.