What is the plot of the house of Hade

The House of Hades by Rick Riordan is a tale of good vs. evil. This book is the sequel to The Mark of Athena in the Heroes of Olympus series. As would be expected, it contains numerous elements of Greek mythology. Seven modern day demigods are the teen protagonists who set out to stop the evil Greek goddess Gaea, commonly known as Earth, from waking and destroying the world. Each protagonist plays a different role in thwarting Gaea's plan. Since demigods are lesser gods who possess divine and mortal qualities, each realizes that their survival is not guaranteed.