While visiting her friend Ingrid in Köln, Yoko finds herself thrust in the middle of a mystery when a young Japanese woman is attacked in the cathedral, and whispers a few words before passing out—notably Rheingold, the German name of a Wagner opera.

Having decided to investigate, Yoko finds signs that the victim had probably set an explosive charge somewhere, and discovers that Rheingold is also the name of... a very special train!