Dark, dank and claustrophobic adventure set on a World War 1 German U-boat with the already precarious life made even worse by Deep Ones somewhat annoyed by the turmoil underwater warfare brings - all those torpedoes and depth charges disturb them, you see. And then they find an ally aboard...
The Investigators take on the roles of members of the submarine's crew, and pre-generated characters are provided. It's likely this will be a one-off adventure, so it is probably best to use them. There is some excellent historical background on German underwater warfare and the U-boat service to help you set the scene (this can be shared with players), and enough explaination for the Keeper as to what is actually going on. It will help if the whole group has a good feel for life on an early submarine, so be sure to discuss everyday details with them before and during play.
The actual scenes for the adventure are well-paced and designed to bring out the claustrophic nature of life aboard an early submarine. If you're looking for background, hunt down a copy of the 1981 TV series Das Boot - although that was a WW2 submarine, it has the same sort of atmosphere you are aiming at. Whilst many events are inward-looking, concentrating on what is going on aboard, there are external events including enemy ships on the surface, an abandoned sunk U-boat and strange noises from outside the hull. Even whalesong sounds spooky if you are in a submarines...
Playtest notes suggest that this adventure can very easily end in a bloodbath which nobody survives, although some smart - or lucky - groups did defeat all challenges and emerge victorious. It seems that those groups where one of the Investigators is the Kapitan of the U-boat have a greater chance of survival; you may prefer this route if you'd rather not kill any more PCs than you have to!
If you want a tight, atmospheric evening where Mythos involvement blends naturally into events, look no further.