Initially I was dubious. Not about the story, the loss of Kiplings son has been the subject of much conjecture, but about the casting.
Let's take Radcliffe, as the most famous. I'm sure most of us have been less than impressed with his acting in HP, one of the major gripes of the film series. Then we have Haig, and in my ignorance I only know him as Inspector Grim in 'The Thin Blue Line', in which he was by far the funniest in it, and worth buying the series, just for him. But, I couldn't see him doing this kind of serious acting. In fact, when I first started watching the film, I found it hard not to chuckle at many of his expressions. As for Cattrall, I don't really know who she is.
Yet I was astonished by them all.
The well documented story goes that Kipling, a patriotic zealot, inspired a call to arms for many young British men. But his own son could not get into the Navy due to poor eyesight. Eventually his father, desperate for his son to play some part, wrangles a way into the army as an officer. In the Battle of Loos, the day after his birthday, he is sent over the top, with his ragtag soldiers. But then nothing is heard of him, prompting the Kipling family to desperately try and discover what happened to him.
As well as being a simply story, it is also a commentary, as all these films are, of the horrors of war, the desperate waste of so many people dragged into it, and the grieving of the loved ones, left at home, with no news. It also highlights the dangers of nationalism, especially when it is be used to encourage men to sacrifice themselves for a fruitless cause (something relevant today with ongoing military campaigns).