We again join Cosmo Mars, the 1950's teen detective, as Steffi and Matt drop into another mystery. This time, they are aboard a Mediterranean cruise ship, in hot pursuit of the notorious Mr X, who has caused mayhem all over Europe. But before they can find their suspect, they must solve the mystery of the woman who vanishes on their first night... Who is the disappearing Miss Lacey? And why is she proving so difficult to trace?