The film is adapted from the best-selling book of the same name by Damian Lewis, a British historian of World War II, "The Ungentlemanly War Department: How Churchill's Secret Warriors Ignite Europe and Give Birth to Modern Black Ops". The film tells the true and bizarre story of the secret combat unit led by Churchill. This small unit used unconventional and completely "ungentlemanly" combat techniques to fight the Nazis, thus changing the course of the war. The story takes place in 1940 during World War II, when Germany continued to bomb Britain and used U-boats to destroy Britain's supply lines. Some British government officials advocated surrender, but Churchill was determined not to admit defeat, but to change the rules of war and fight back strongly. According to intelligence, the maintenance, repair and arming of the German Atlantic U-boat fleet were all carried out in a small port in Nigeria. Churchill saw an opportunity to destroy German war assets and regain the advantage. However, he realized that a direct attack on this Spanish colony might lead to Spain's alliance with Germany, so he formed a secret combat unit led by Captain Gus March Phillips (Henry Cavill) to launch a secret operation to destroy the fleet. When the initial plan to blow up the fleet was thwarted by the sturdy ships and shallow harbor water, the squad quickly devised a plan to steal the ships. Their daring operation was successful, destroying a vital part of Hitler's war machine and reopening supply routes to Britain, thus hastening Germany's defeat.