Through assiduous research and well-paced narrative, Simon Parkin has given us an extraordinary, little-known story from World War II — the turning point in the Battle of the Atlantic. Its subjects are the brilliant and creative Gilbert Roberts and a team of young women whose strategic war games simulations uncovered why German U-boats were so adept at taking out British ships in the early years of the war — more than 1,200 in 1940 alone.

This work of nonfiction reads in part like a thriller, but it is the reader's thrill to learn about Roberts, his luminous mind, persuasive narrative gifts and the many setbacks he endured before being called back to service in 1940. The Royal British Navy, which had relieved him of duty when he contracted tuberculosis, called him back to conduct training exercises in 1940, but he was upgraded the next year to a task that would prove his tactical prowess. He was charged with figuring out how the U-boats were succeeding, devising a strategy to stop them and imparting the strategy to all British naval vessels.

Among hundreds of young women who had joined the Women's Royal Naval Service, Jean Laidlaw, Jane Duncan, Janet Okell, Nancy Wales, Christian Oldham, Judy DuVivier, Elizabeth Drake, Elizabeth Hackney, Jane Howes, Doris Lawford and Pauline Preston were chosen for their math and tactical skills to work with Roberts.

The women, some in their late teens, were dubbed Wrens — "my Wrens" — by Roberts. He and the Wrens created a Battleship-like game that used after-battle reports, detection systems, board game simulations and help from the Enigma machine, whose encrypted messages by the Germans were famously cracked by mathematician Alan Turing.

Karl Doenitz, an admiral who became the German head of state briefly after the war, had been a U-boat ace during World War I and was in charge of creating designs and tactics that could help Germany win World War II. He argued for and got more and more U-boats that could amass like a pack of wolves tearing into entire convoys. British ships had no radar to detect submarine activity and only some had an early sonar device that sent out sound waves. When the "ping" wave hit a solid object within a 3,000-meter radius, it registered a "tong."

One tactic employed by several U-boat commanders was to take their crews inside the columns of the convoy to attack, evading detection from sonar, which projected outward, not under. Rations were on a countdown clock of months and weeks as supply ships were targeted and sunk at alarming rates. British leaders feared their people would starve and the war effort be grounded for want of fuel and other supplies. Something had to be done at sea.

Renowned for his tactical brilliance, Roberts became head of the Western Approaches Tactical Unit in Liverpool, where he and the Wrens worked long hours, sometimes plotting sea movement strategies into the night. They eventually refined a game and its variations, with names like Raspberry and Half-Raspberry.

At the end of the game, the players sat in a circle and rehashed what happened. Once they had the game truly seaworthy, playing it over and over and over, Roberts and the Wrens demonstrated it to Adm. Percy Noble, commander-in-chief of Western Approaches, who had been condescending and skeptical of Roberts' assignment. As he watched the demonstration, Parkin writes, "he and his staff seemed to sit forward in their chairs."

Noble congratulated Roberts and asked a member of his staff to inform the prime minister about "a cardinal error in anti-U-boat tactics. A new immediate and concerted counterattack will be signaled to the fleet within 24 hours."

The new Western Approaches convoy instructions became "a bible issued to all British escort officers," as well as American and Canadian vessels, Parkin writes. Before leaving the room, Noble turned to Roberts and said, "Sew on your fourth stripe."

Roberts held his Wrens in high esteem for their skill and intellect, and his detailed diaries of the work they did together was "a one-man campaign for posterity," Parkin writes. It's startling to imagine that, had Roberts not been so conscientious, he and the Wrens would have been lost to us.

Diana Nelson Jones is a writer for the Pittsburgh Post-Gazette.