The Fraud Squad of the Metropolitan Police has noted that some impressionist paintings of doubtful provenance have been turning up at auctions within UK. The suspected source of these fakes is the South of France with the River Rhone a possible South to North transit route. Tom Anderson, a rookie undercover detective is sent to see what he can discover about this lucrative scam. He joins the river boat Voyager at Lyons on a trip which is advertised as a holiday for singers with a public performance in Avignon. As Voyager sails down the Rhone, Tom is watching the other passengers while appearing to enjoy the choir rehearsals in preparation for the concert. The seemingly harmonious atmosphere on Voyager soon deteriorates as suspicions mount and Tom finds himself trying to second guess some ruthless characters.