Picking up where his 2013 novel Dream On left off, What I Know I Cannot Say follows the life story of Billy's father, Dai Maddox. When Billy's former partner Bran shows up wanting to record Dai's life story to put together a documentary, Dai looks back on his past, remembering his childhood as a destitute orphan, his work as a collier in the mines and the subsequent drifting between menial jobs, alleviated only by reading and drawing; his enrolment in the British Army and participation in the invasion of Italy during the Second World War; and post-war life under socialism, when he was back in the pits and married to Billy's mother, Mona.

Moving from the heyday of the pre-mechanised coal industry to the present day, What I Know I Cannot Say presents a moving and vivid panorama of twentieth-century Wales, brought to life by Smith's meticulous attention to historical detail and a distinct gift of invoking the smells, sights, and sounds of the past. We find ourselves smelling the cordite of ammunition among the ruins of Cassino in 1943, during the invasion of Italy; the damp coal in the mineshafts; the beer-soaked wood of pub floors; the smell of fresh coffee from a modern percolator. Dai's journey is emotional and moving, told in gritty, realistic prose.