While limitations of available film which could get reliable images in lower lighting conditions prevented this film from including "below decks" footage which would have made the portrait more well rounded (see the marvelous if fictionalized portrait of WWII submarine life in DAS BOOT!), this German film, repackaged after the war for English speaking audiences (adding a final scene of the surviving German ships turned over to the British at the end of the War, including the largest submarine of the day which had been a later command of one of the U-35's commanders) is essential viewing for fans of maritime history as well as specific submarine enthusiasts.
The "combat footage" deals only with the final boarding and sinking of the U-35's victims on the cruise documented (it was reputed, on fairly good evidence, to be the single most successful submarine in WWI service) the inter-cards acknowledge the long battles in some cases preceding these denouements. This may strike some younger viewers as odd if they are not aware that prior to the British Admiralty's ordering hidden gun emplacements on the decks of their merchant ships to ambush submarines which rose to challenge them and forcing them to abandon such gentlemanly tactics, most submarines followed the then accepted Rules of War and did not sink ships without warning.
Both as a portrait of the state of submarine life (as much as could be shown above decks) and warfare midway through WWI, this is one of the best films available. Highly recommended.