3年前,在多瑙河畔的一次度假旅行中,一名游客在维也纳附近神秘地失踪了。有人认为他掉到河里了,可是河里没有发现他的踪迹。也有人认为,他可能被同行的另一名游客谋害了,但是又没有证据。警察怀疑一个人有谋杀动机,在搜查他的卧室时发现,他靴子上的泥很多,于是带了一点样品给维也纳大学的威廉·克劳斯教授化 Three years ago, on a holiday trip by the Danube, a visitor mysteriously disappeared near Vienna. Some people think he fell into the river, but the river did not find his trail. Others believe he may have been murdered by another fellow traveler, but there is no evidence. Police suspect that a man was motivated by murder and found a lot of mud on his boots while searching his bedroom and brought a little sample to Professor William Klaus of the University of Vienna