This book belongs to the series 'The Heroes of Olympus.' It's the second book in the series after 'The Lost Hero.' It's about a demigod named Percy Jackson who lost all his previous memories because of Hera's effort to unite the Romans and the Greeks to fight the earth goddess Gaea. She exchanged the demigod leaders of both camps and put them in each others places. Percy eventually meets two new friends Hazel and Frank. All three of them go on a quest to rescue the death god Thanatos in Alaska, kill the monster Polyphemus and try to close the doors of death so the monsters don't escape from the underworld anymore. With Percy's water controlling power, Hazel's controlling underground riches and Frank's ability to turn into any animal, they were successful in saving Thanatos and win against Polyphemus.Later, they find out that they are in a prophecy that includes seven demigods. They get a message that the other four demigods were arriving with the warship to pick them up so they could go on a bigger quest to defeat Gaea and put her to sleep forever.