Barton (Brian Deacon), a soldier living in England during WWII comes across Alice (Glenda Jackson) who has been keeping her husband's farm going single handed whilst he is being held a prisoner of war. Soon Alice and Barton become lovers but to protect Alice from gossip as well as the fact he decides not to return to the army Barton agrees to wear a dress and makeup so he can pretend to be her sister, Cathy. And the locals are convinced as is a brutish army sergeant (Oliver Reed) who when he comes across the farm takes a shine to both women. Flattered by the sergeant's attention Barton agrees to go to a dance with him being naive to the consequences.
"The Triple Echo" is an intriguing little movie which has this evolving scenario which in turn causes the character dynamics to change. As such what "The Triple Echo" becomes is a look at Barton and how things alter for him as he goes from a young soldier who falls for a married woman to living life as a woman, feeling trapped by the pretence and finding himself unexpectedly flattered by the attention he receives from a sergeant. Whilst this is going on we also see how living a lie affects Barton's relationship with Alice as at times she is no longer attracted to him whilst resentment creeps in as she has to feed two mouths on what little she has.
All of the above makes for an intriguing movie but one which then unsurprisingly turns disturbing when the Sergeant takes Cathy to the dance and inevitably discovers the truth. I won't go in to detail but you can imagine up to a point how this plays out especially with Oliver Reed in full intimidation mode from the first minute we meet him, although it may not end exactly how you expect. And it certainly leaves an impact on you but rather strangely not to the point that you find yourself wanting to watch it again.
What this all boils down to is that "The Triple Echo" is genuinely a very intriguing movie with this character dynamic which evolves and keeps you interested. But it is a watch only once sort of movie as it doesn't have the sort of complexity which makes you need to watch it again.