This is UC61 stranded on the continent's shores by its crew. Summer of 1917 had unterseebootes like UC61 attacking Allied shipping in the Channel. This U-boat avoided the fiercest dangers from the Brits but ended up running aground on a sandbar. The Belgians captured the crew and preserved what they could as the Germans had begun destroying documents and equipment when they abandoned the boat.
The Brits found out the Germans had cracked their codes. The Brits then turned around and broadcast false information. This helped them as another U-boat sank in August by a mine that was laid by another German sub, but the Germans thought due to transmission that the Brits had swept the mine away.