Shortly after the beginning of the war in 1939, the Germans are laying mines by submarine and airplane all over Britain's sea lanes and in the estuaries. They're contact mines, the ones that look like cartoons or viruses with multiple spikes. They can be disabled by the usual methods of paravanes that cut their cables and bring them to the surface, where they are destroyed by gunfire.
But than a new type of German mine is introduced, more deadly. It lies on the bottom and doesn't require contact. It's called an influence mine because it is detonated when it somehow detects a ship overhead -- by sound or magnetic signature. Several are found in shallow water but they all explode before they can be retrieved.
Much of the program is taken up with the first example that the Brits are able to reach on a mud flat. Not knowing exactly what they're dealing with, a British expert and an Able Seaman slowly dismantle the horrid thing. It turned out to be a magnetic mine. The technology lost me at times. The chief impression this long sequence left with me is that I wouldn't have done what these two guys were doing.
It become less technical and more informative after that, in describing the various kinds of counter-measures deployed. The graphics help a good deal.
Like the British, the Japanese were an island nation dependent on imports. The Americans deployed many influence mines similar to those first used by the Germans.
The war ended, dramatically, tragically, unavoidably.