Once Upon a Crime...

Phoebe and a fellow American in Rome find a dog with a $5000 reward. They take a train to the owner in Monte Carlo. She turns up murdered. They run and become suspects just as 3 other Americans on the train.
On their way to Monte Carlo, Monaco, Marilyn and her husband, Neil, meet several other married couples, including Julian and Phoebe, who are traveling with a lost dog they plan to return to its wealthy owner for a large reward. But, when the dog's mistress is murdered, the travelers become the prime suspects, and Inspector Bonnard is determined to track them all down.