Demigod Percy Jackson, still with no memory, and his new friends from Camp Jupiter, Hazel and Frank, go on a quest to free Death, but their bigger task is to unite the Greek and Roman camps so that the Prophecy of Seven can be fulfilled.

When Neptune tells Emily and her merman father and human mother to return to Brightport to try to make merpeople and humans work more closely together, Emily faces problems with old enemies, her new, half-merfolk friend Aaron, and a mystery related to a group of legendary lost sirens.

Neptune, King of the ocean, sends half-mermaids Emily and Aaron to the land of the midnight sun to investigate the source of the dangerous storms brewing in the ocean.

Despite Emily and her boyfriend Aaron promising their best friend Shona a fun, adventure-free vacation on a relaxing tropical island, adventure finds them anyway. As semi-mers, Emily and Aaron discover an island that only they can get to, where they learn of an imminent earthquake that threatens the entire region. They must then fulfill a prophesy and find the giant who is supposed to save them.