Sam Carter (Nigel Patrick) is a jeweller's clerk who dreams of stealing a fortune in diamonds and eventually does so but kills a man in the process. He then embarks on the highlife but is pursued across Europe by Milo March (Jack Palance) a private detective who suspects that not all is above board. However, Milo is not alone in his pursuit as Trudi Hall (Anita Ekberg) has her own ideas as to how the money would be best spent. Also two thugs, Martin Lomer (Bonar Colleano) and Gerard Heinz (Robert Stone) are after the Largest Diamond in the stolen hoard. All of the main characters end up fighting and trying to outwit each other over the Largest Diamond worth $700,00 on a Train going to London. This Precious Diamond is described by Milo March (Jack Palance) as: "$700,000 of unhappiness" because people are willing to do anything, even kill, to get it.