Robert Langdon is in Italy. He wakes up in a hospital with no memory of recent events. He appears to have suffered some head trauma. He's being treated by Dr. Sienna Brooks. Suddenly someone dressed in police uniform comes in and starts shooting, trying to get to Langdon. Brooks helps him.
Langdon discovers he received an email from a friend in Italy who told him that he too is being hunted, and he's gone into hiding with what they took. Of course he has no idea what he is talking about. He finds something in his belongings that reveals the Map of Hell which is inspired by Dante's Inferno.
He learns about a man Bertrand Zobrist, a billionaire who believes the world is too overpopulated and that drastic measures are needed. He plans to release a virus which he created to kill half the planet's population. Evidently he's inspired by Dante so Langdon needs to find out what he is planning, while he's being pursued by people from the World Health Organization, and one of them is someone he knows.