One passenger goes missing each day the ship Royal Holiday is in port on its 15-day Mediterranean cruise, and that's not good for publicity. An Interpol agent is brought on board to help investigate, and he thinks he sees a numbers pattern.

The pattern is similar to the one discovered by Mathematics Professor Alfred Dunningham, PhD, who solved the mystery of the murders in Sydney, Australia.

The pattern alone isn’t enough to solve the mystery, so the cruise line (with Interpol's help) locates the Professor in Paris, France, where he was being honored for solving the murders in Sydney.

Professor Dunningham agrees to come to the ship to help them. There is some similarity to the Sydney case, but the Professor dismisses that notion — “Too obvious; would look like a copy cat,” he says.

The cruise is almost over; can he solve the mystery before the end of the cruise?