So the pretext is that in the context of Operation Alberich, where the Germans fell back from two salients that they seized to the Hindenburg Line, a British general who has for some reason decided to put his headquarters at the front lines at the end of one of these salients, decides to call off one little isolated attack. But because the general is in a bad position to communicate with troops under his command and because the Germans have magically destroyed all the telephone lines, the general decides 1. Wait for an hour or so for the telephone lines to be repaired and then calls off the attack 2. Sends a runner back to headquarters to which sends a message by telephone to a place where they can send a runner to call off the attack. 3. Sends their driver (they are a general, they would have a driver) to deliver the message via the roads around the area. 4. Relocates their headquarters to a central location where they can communicate more easily with troops under his command. 5. Use a set of runners to run behind the lines in shifts to get the message to call off the attack to the troops. 6. Sends 2 soldiers to cross deadman's land and the former German lines, then cross British lines without being shot (ummm...they actually skip that bit in the movie) and deliver the order.

If you chose 6, then you are an idiot and will enjoy this movie and have a good shot at being a Hollywood hot shot. Otherwise, just enjoy the acting and cinematography of this movie where very little makes sense.