Alastair Denniston was a very important man during World War II. He was an officer in the British Navy, and he was in charge of a secret project called "Ultra." Ultra was a very clever plan to intercept secret messages from the enemy (in this case, the Germans) and decode them. This allowed the British to figure out what the Germans were up to and what their plans were. Alastair Denniston made sure that Ultra was successful, so that the British could stay one step ahead of the Germans and protect their country.