Book excerpt: Could the killing of Germanicus Julius Caesar—the grandson of Mark Antony, adopted son of the emperor Tiberius, father of Caligula, and grandfather of Nero—while the Roman Empire was still in its infancy have been the root cause of the empire's collapse more than four centuries later? This brilliant investigation of Germanicus Caesar’s death and its aftermath is both a compelling history and first-class murder mystery with a plot twist Agatha Christie would envy.