Dennis (Mark Burns), a British agent of some sort, is on "the island" (Malta but it's not named) to carry out an undercover investigation of two crooked casino owners. The suspicion is that they get guests who run up gambling debts to become the sort of smuggler Customs won't suspect. Dennis runs up a debt in the hope he'll be induced into the racket.
The film opens with the murder of one of owners by Francesca (Patsy Ann Noble), a femme very fatale indeed, and her lover Joe (Shaun Curry ), who want all the ill-gotten gains for themselves.
The other owner, Malo, is found dead just after he'd given Dennis an advance in return for his passport. The local police naturally suspect Dennis. The thing is Malo was found in a locked room, seven floors up with no sign of the murder weapon. The solution to this locked-room mystery is about as good as this film gets.
Priscilla (Wanda Ventham) is sent out from the UK to help Dennis while posing as his fiancé. So far, so good. But that's as good as it gets. We get to see sunny skies and sparkling seas, we get to see another of Joe's girls topless in a scene that seems to be included because they'd an actress who'd go topless back in 1966 or maybe to get a 1960's X rating, we get to see the good girl and the bad girl in their bikinis, and - not much else. The film is padded out to barely feature length with Anita Harris singing a song, multiple sequences of our hero darting down side streets trying to dodge the most visible police tail in history and of our villainess swimming underwater.
Cut out the padding and the topless scene and you'd have had a good hour-long episode of a Sixties TV series.