Mark Twain was one of the greatest minds of his time, torn between the brilliant persona he had forged for himself and a life of wrenching tragedy. Nicola Tesla was an unworldly genius capable of insights that defied the wildest imaginations. Their secret history is rife with friendship and betrayal, human tragedy and unearthly danger. Drawn by his curiosity, Samuel Clemens escapes the grinding toil of being Mark Twain by cultivating what seems an innocent friendship with the greatest scientist of the age. As he grows closer to the powerfully eccentric Tesla, he begins to sense another, stranger intelligence that may be coming into being. The inventions of Nicola Tesla—alternating current, wireless communications, death rays, robot weapons—become puzzle pieces that take shape under Mark Twain's eyes. Has Tesla somehow opened the gateway to a profoundly alien intelligence, or is it Tesla himself that will bring the world to Armageddon? And with every tragedy in his family—buffeted by the deaths of his wife, daughter, brother, and son—Samuel Clemens is moved to ask the most important question of all: Why is the world worth saving?