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.

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.

"When King Neptune uses the last remaining drops of magic mermaid ink to create creatures designed to put NYC underwater, Eric "Doodles" King calls upon his Coney Island friends, aka Monster Club, and his drawing skills to stop Neptune before it's too late"--