Actually, the protagonists come upon the invasion plan in an advanced state of preparation; but still with a lot left to do before it could be implemented. One of the crucial factors for its success is to make sure that the lighters being built for troop transport would actually stand open sea conditions - unlike the ordinary lighters used in this area, which usually do not venture out of the protected waters behind the Frisian islands. The clue followed by Carruthers, in a brief overheard conversation of the German plotters, is that one such lighter would be tested under open sea conditions and that a certain high-ranking German "insisted on being personally present". The hidden Carruthers is present at the successful testing voyage when the lighter proves fully seaworthy, and then manages to sabotage the tug and escape. In the confusion Carruthers sees the face of the high-ranking participant and recognizes him as "one who, in Germany, has a better right to insist than anyone else" – the clear implication being that it was Kaiser Wilhelm II in person.