"The Lost Symbol" is a mystery thriller novel by Dan Brown, featuring the character Robert Langdon, a Harvard symbologist. The story unfolds in Washington, D.C., where Langdon is summoned to give a lecture at the United States Capitol. Upon arrival, he discovers that his mentor, Peter Solomon, has been kidnapped, and a mysterious symbol is left behind. As Langdon races against time to solve the clues, he delves into the world of Freemasonry and ancient secrets. The novel explores themes of knowledge, power, and the quest for enlightenment, while intertwining historical references and symbols throughout the narrative.