(M) ****

Director: Morten Tyldum.

Cast: Benedict Cumberbatch, Keira Knightley, Matthew Goode, Mark Strong, Charles Dance, Allen Leech, Matthew Beard, Rory Kinnear.

CERTAIN things happen when true stories get turned into films.

Real people become characters with easily defined and exaggerated traits, while real events get condensed, altered and just plain made up in order to make the realities of life fit more easily into the typical three-act structure and dramatic tension we expect in a movie.

The Imitation Game is a perfect example of that, and therein lies the film's only weakness - by squeezing this story of World War II codebreakers into the required formula, it tends to feel contrived, relying on clichés to tick the necessary narrative boxes.

But otherwise, this is a powerful and fascinating fictionalised account of the life of computing pioneer and bona fide genius Alan Turing, driven by Cumberbatch's engrossing performance in the lead role.

Skipping back and forth between three time periods, the story examines Turing's formative school years, his work cracking the Nazi's Enigma code during WWII, and his later persecution for being a homosexual in an era when such a thing was still illegal in the UK.

The main focus is the war years, where Turing is portrayed as having an Asperger's-like demeanour that puts him at odds with his fellow codebreakers (played by Knightley, Goode, Leech and Beard) as he struggles to build the machine that he believes will break the code and help win the war.

It's Cumberbatch's performance that dominates and powers the film. He is utterly mesmerising and, among a growing collection of excellent turns, this is his finest to date.

Also great and potentially in career-best form is Knightley, whose role as fellow cryptanalyst and Turing's fiancée Joan Clarke is overshadowed by Cumberbatch but no less important or impressive despite its comparative lack of dramatic fireworks. A top-notch supporting cast, particularly Goode, Strong and Dance, round things out nicely.

In general this is a fantastic story well told. With its themes of secrets and what defines a person, set against a backdrop of WWII intrigue and topped off with a sadly real coda about the persecution of homosexuals in a less enlightened time, The Imitation Game is packed with plenty of interesting ideas and plot elements.

Where it falls down is in its overly obvious efforts to fit the true story into something resembling the traditional flow of a movie. As a result we are left with such tired clichés as a race against the clock, a eureka moment, and an "if he goes, I go" speech, all of which genuinely feel out of place and lead to some moments of ham-fisted melodrama that are only saved because the cast is so damned good.

There's also the non-linear presentation, which creates some pacing issues and strangely timed reveals as the plot dives in and out of Turing's school years, war years, and post-war years.

At least Tyldum (best known for the Norwegian actioner Headhunters) and cinematographer Oscar Faura (The Impossible) give each period a subtly different look, and the direction overall is effective, even if its occasional brief war sequences are largely unnecessary.

Generally though, The Imitation Game is impressive, particularly due to Cumberbatch and Knightley and the highs and lows of Turing's truly remarkable life.