A moving tale about mortality - one contemporary; one in the trenches of World War I

Sam is sixteen, and at war with his own body, fighting an incurable illness that gives him only months to live. Time has suddenly become very important to him as he now has so little left. Nearly a hundred years ago, another Sam - a lieutenant in the British Army - is off to fight a different kind of war, on the Western Front. He knows that he may well not survive. Linking the two is a girl named Marion. But is Marion just a figment of Sam's imagination - a hallucination caused by his medication - or something far more extraordinary? Could she somehow be. a bridge across time?