THE psychological ramifications of war make for engaging and thought-provoking viewing when done properly (Born On The Fourth Of July, The Deer Hunter and Apocalypse Now all feature characters with PTSD symptoms).

Legacy attempts to get into the mind of a combat veteran who has done unspeakable things but while the premise is interesting, it falls short in delivering a satisfying cinema experience.

Idris Elba stars as Malcolm Gray, a decorated soldier part of an undercover team codenamed Dark Hammer, who specialise in pre-emptive strikes against civilian targets, all in the name of freedom, democracy and other such ironic ideals.

After finally tracking down a Ukrainian with a stockpile of sarin gas, the operation goes badly wrong resulting in his squad’s capture and his horrific torture.

Back in the USA, he hides away in a rundown motel room convinced that the government will soon be coming for him because of his botched cover and his connection to his brother, the highly respected Senator Darnell Gray who covertly founded Dark Hammer, and is also expected to enter the forthcoming presidential race.

Holed up in his dingy motel, he’s visited by a journalist, a former lover, his comrades, and an assassin disguised as a pizza delivery man as he struggles with what’s real and what’s in his head.

Idris Elba is a brilliant actor. His presence as Stringer Bell in The Wire is one that will be long remembered, but sadly, his film career so far has been less than spectacular with a string of one-note performances in movies like Takers and The Losers. Legacy isn’t going to do any favours for his CV.

His performance is admirable, staring into the middle distance; twitchy at every little movement, it’s believable and unnerving.

But he can’t carry everything and for a film that relies heavily on dialogue and takes place almost exclusively in one room, it’s terribly written (sample: “a conscience will drop you faster than any bullet”).

The film hinges on the blurring of reality and fiction. Are these people real or figments of his imagination? But it’s so obvious that they can’t be real from the outset that when Legacy pulls its big reveal: that everything has been in his mind all along, it’s no revelation at all.

It’s like unwrapping a bottle of wine – you might not know exactly what it is, but you can see the shape of it a mile away and you’d have to be pretty slow on the uptake to be surprised when the wrapping’s off.

While Elba’s performance as a solider with Post Traumatic Stress Disorder is convincing and at times disturbing, Legacy’s reliance on tired visual devices to show his unravelling mind (head butting a mirror to leave a fractured reflection, a constantly ringing telephone) are wearing, and it’s crippled by its dialogue which lends no credibility to its supporting cast.

Legacy: Black Ops (15) is released in cinemas today.