Interesting and informative, as all the episodes in this series have been.
Most of the time is given over to the Battle of the Atlantic, which pitted German U-boats against Allied convoys and their scant escorts. The advantage shifted back and forth. By late 1942 it looked as if England, an island nation, would be starved of food and supplies from the US and Canada. In May, 1943, the German effort lost out to Allied technology and to the weight of numbers. By late 1944, submarine patrols had become suicide missions.
Also covered are the part played by Italian and British submarines in the Mediterrean. In northern Africa, Rommel's Afrika Korp was completely dependent on supplies shipped from Italy. Having broken some critical codes, the British knew when and where the transports could be found. Rommel lost the battle in Africa in large part because he ran out of supplies. It's not mentioned in the film, but in one case, a tragic irony, Britain ordered an air attack on a small Germany convoy. As the airplane approached the ships, it was learned that many Allied prisoners were aboard, but the attack couldn't be aborted without signaling that the codes had been broken.
In the Pacific Ocean, both the Japanese and the Americans had long supply lines. The American submarines, like the U-boats, were ordered to concentrate on transports because Japan was an island nation like England. The larger Japanese submarines concentrated on warships. They were skilled and effective but it was the wrong strategy.
Nicely done.