First thing to bear in mind is that the screenplay is based on Jean Bruce's novel and is part of the OSS 117 spy novels ;However the movie (directed by a French and a French /Italy co-production) does not feature Hubert Bonnisseur De La Bath alias OSS 117 :he is replaced by an American secret agent Art Smith (played by Sean Flynn ,whose short career essentially took place in Italy.)Part of the reason can be found in the numerous OSS 117 films which were produced at the time in France ,portrayed by Kerwin Matthews,John Gavin ,Frederick Stafford...before being brilliantly parodied by Jean Dujardin in recent years: Flynn would have been another OSS 117 ,so they changed his identity.Actually Toublanc-Michel was not allowed to use the character :he would have infringed copyrights.
As for the story itself ,it is an implausible story of marines ,mysteriously disappearing:your disbelief must be suspended when you discover what terrible fate lays in store for them;nevertheless,it has become some kind of cult movie ,weren't it only for its title (the word game ,present in all Bruce's titles,"Cinq Gars Pour Singapour" (=five guys to Singapore)which is lost in translation and its extravaganza.A female part (gorgeous Marika Green) was fleshed out and became European.
As for the story itself ,it is an implausible story of marines ,mysteriously disappearing:your disbelief must be suspended when you discover what terrible fate lays in store for them;nevertheless,it has become some kind of cult movie ,weren't it only for its title (the word game ,present in all Bruce's titles,"Cinq Gars Pour Singapour" (=five guys to Singapore)which is lost in translation and its extravaganza.A female part (gorgeous Marika Green) was fleshed out and became European.