On 4 September, the submarine was caught in the Ottoman anti-submarine net as it began another tour. Despite such reverses, by mid-September, Allied nets and mines had closed the eastern entrance to the Dardanelles to German U-boats and "U-21" was thwarted when it tried to pass the straits to Istanbul on 13 September. The first French submarine to enter the Sea of Marmara was but it was forced to turn back; on 30 October, when returning through the straits, it ran aground beneath a fort and was captured intact. The crew of 25 were taken prisoner and documents detailing planned Allied operations were discovered, including a scheduled rendezvous with on 6 November. The rendezvous was kept by the German U-boat instead, which torpedoed and sank "E20", killing all but nine of the crew.