The Unlikely Spy, by Daniel Silva, is the story of a British professor who is enlisted into the British Intelligence Service via his friendship with Winston Churchill and whose actions are instrumental in preserving the secret location of the Allied Invasion in 1944.

The story begins with a murder of an innocent woman in 1938 and heralds the arrival of sleeper spy Anna Steiner into England. Anna is a very intelligent and thorough secret agent who establishes cover in England as Catherine Baker and then waits for six years before she is activated.

Alfred Vicary is a university professor of history who has a longtime friendship with Winston Churchill. In 1940, Vicary is pressed into service as a spy master in MI5 and becomes one of the best in the business at finding German spies. Vicary works for a man called Basil Boothby and is partnered with an ex-London police officer named Harry Dalton. Together with cryptologists, radio listeners, and watchers, these men are very successful in capturing spies. Yet as the Allied forces begin preparation for an invasion, Vicary learns that a spy network MI5 knew nothing about is being activated, and present a danger to the success of the invasion.

With Boothby manipulating the facts and pushing Vicary for results, the team of Dalton and Vicary find themselves racing against time to uncover these sleeper spies and prevent them from reaching their German controllers.

Sean Dogherty is one of these sleeper spies, and he assists Horst Neumann with entrance to the country and a cover at his small farm in the North of England. Jenny Colville, a young woman who often seeks shelter with the Dogherty family when her father is violent, falls in love with Neumann. Sean's wife Mary is not happy that Sean is a spy but does nothing to stop him.

Anna and Neumann make contact in London and Anna learns she is to seduce Peter Jordan, an American engineer who has been tasked with helping the English create the Mulberries or giant concrete structures that will be used in the Allied invasion.

Knowing that the Mulberries will be sunk and used as breakwaters to assist in the invasion is the key to knowing that the invasion will be at Normandy and not Calais. Compromising Jordan, then, becomes the key to the Germans finding out where the invasion will be located.

Anna seduces Jordan and is successful in retrieving the information. As she prepares to transmit this information to the Germans, she also realizes that she is in danger of discovery and arranges to be taken out of England. Vicary and Dalton have indeed figured out who she is and the chase begins.

Neumann is able to extract Anna from London and in a stormy English night along bad roads, the two make their way to Dogherty's farm. Jenny's father is killed there and so is Sean. Jenny is forced to come with Neumann and Anna because Neumann feels sorry for the girl.

Vicary and his team are in pursuit and when Anna, Neumann, and Jenny board a fishing vessel and head to sea to meet the U-Boat awaiting them, the British are not far behind. Jenny is able to break the engine on the fishing boat, and as the British catch up the two spies are killed before they can board the U-Boat and give away the information about the location of the invasion.

After the war, Vicary is dismissed in disgrace from MI5 but he soon works out what happened. Vicary asks Boothby to visit him at his home in Gloucestershire and learns that has been manipulated by Boothby since the beginning of the operation with the German spies but it was for the good of the country. Vicary retires to his life in Gloucestershire, content that he is no longer just a professor of history but actually a part of it.