This parable of the last days of civilisation as we know it takes place mostly underground, as the survivors of what seems to have been an accidental missile exchange wait for each other to die. The letters of the title are interior monologues by an elderly scientist (Bikov), addressed to the son he knows must have perished amid the briefly glimpsed devastation on the surface. What is most remarkable to find in a Soviet film, apart from the resolutely unpartisan pessimism, is a clear religious thread. The band of silent children who represent the hope of the future are initially in the care of a priest, before the dying scientist takes upon himself their salvation. Old hands will detect shades of Tarkovsky in this; in fact, Lopushansky was assistant on Stalker. It may not be a masterpiece - it's often static and rhetorical - but it is a humane and timely film, and few will resist the sheer emotion of its ending.