In November 2019, a London teenager named Zac Brettler plunged from the fifth-floor balcony of Riverwalk, an expensive luxury apartment building along the north bank of the Thames River. After weeks of speculation and mystery surrounding his death, closed-circuit television footage from the nearby headquarters of MI6, Britain’s foreign intelligence service, would reveal that he had jumped. But why? This question preoccupies London Falling, a masterly if eccentric and at times frustrating work of investigative journalism by the New Yorker’s Patrick Radden Keefe.