The New York Times presents a documentary audio series that follows Rukmini Callimachi, a foreign correspondent for The Times and a frequent voice on “The Daily,” as she reports on the Islamic State and the fall of the Iraqi city of Mosul. With the producer Andy Mills, Rukmini journeys to the heart of the conflict to grapple with the most pressing questions about ISIS and to comprehend the power and global pull of the militant group. Today, instead of our usual show, we offer the Prologue and Chapter 1 of “Caliphate.” For more information about the series, visit nytimes.com/caliphate. This episode includes disturbing language and scenes of graphic violence.