"Gregor and the Marks of Secret" is a fantasy novel by Suzanne Collins, published in 2006. It is the fourth book in the "Underland Chronicles" series. The story follows the adventures of a young boy named Gregor, who finds himself transported to the mysterious world of the Underland, a hidden civilization beneath New York City.

In this installment, Gregor and his little sister Boots are called back to the Underland to help solve a mystery. Strange marks have been appearing on the walls of the Underland, and the humans and creatures living there are unsure of their meaning. Gregor is tasked with deciphering the marks and uncovering their purpose, all while navigating the dangerous politics and power struggles of the Underland.

As Gregor and his companions investigate the marks, they uncover a sinister plot involving a secret society that seeks to overthrow the ruling regime of the Underland. With the help of new allies and old friends, Gregor must use all of his skills and knowledge to stop the conspirators before they can bring about a devastating war.

Throughout the book, Collins explores themes of loyalty, trust, and the consequences of power. Gregor must learn to trust his instincts and rely on the people around him in order to succeed in his mission. Meanwhile, the complex relationships between the various factions of the Underland are explored in depth, as Gregor struggles to navigate the ever-changing political landscape.

Overall, "Gregor and the Marks of Secret" is an exciting and engaging read, with plenty of action, suspense, and emotional depth. Fans of the previous books in the series will not be disappointed, and newcomers will find plenty to enjoy in this richly imagined world.