Morgan Freeman, Rebecca Hall, Cillian Murphy and an AI Johnny Depp Warner Bros

Few directors have debuted with as much fanfare, star power and budget as Wally Pfister, the former cinematographer of Christopher Nolan who has flown the coop to tackle his own project based on a 2012 Hollywood Blacklist script by Jack Paglen.

A sort of modern day techno-thriller Frankenstein, Transcendence follows Johnny Depp's rock star artificial intelligence researcher Dr Will Caster who, after a run-in with anti-technology extremists, finds himself with days to live.

In a bid to save him in some form, Will's girlfriend Evelyn (Rebecca Hall) decides to transfer the electrical currents of his brainwaves to a super-computer – essentially uploading his consciousness. It works, but is what they've created really Will, and why does it instantly ask for "more power"? That can't be good.

Virtual Johnny Depp asking for more power is indicative of a script peppered with clunky little bits of dialogue like "kill the internet" and "uploading a monkey to a computer". They may be hilarious, but such lines only serve to weaken the film's already loose grip on reality.

For the most part Transcendence does well to root itself in a believable, heightened reality and poses many "What if?" questions. It's not a film about the technical details, however, and anyone who spends their time picking the film apart is really missing the point. Transcendence is about humanity, gods and monsters – a cautionary tale, but one more concerned with entertaining than lecturing.

Johnny Depp's dying Will Caster gets plugged into the mainframe Warner Bros

Despite being on screen more than most, Depp is decidedly bland as the lead. The initial thrill of seeing the actor in a relatively restrained role (despite him becoming the internet) is lost as he mumbles his way through the first half before deadpanning his way through his scenes as a rampant AI.

Likewise Morgan Freeman, Cillian Murphy and Kate Mara are given nothing to work with. Murphy is a default FBI agent and Mara little more than an ideology with blonde hair and mascara. Only Freeman gets away with it because as always he is a welcome screen presence.

It's Rebecca Hall as Evelyn and Paul Bettany as friend Max who save the film, providing the heart with two great performances as the grieving other half desperate to keep her lover alive and the academic torn between his scepticism and his best friend. Hall in particular is marvellous in the film's pivotal role.

Another area the film succeeds in is its moral ambiguity. Those extremists at the films start become part of the solution by the end, as their fears are proven right. However even during Transcendence's climax Pfister is perfectly content with making no distinction between right and wrong. Many films make the distinction all too clear but sometimes that goes against the subject matter, as it would have here.

Johnny Depp as Dr Will Caster during a TED-style talk at the start of the film Warner Bros

Great science fiction poses interesting questions about the nature of humanity. Transcendence does this, but it asks those questions knowing full well the answer – whatever it is – is murky. What it does offer up is a set of vaguely believable consequences when the principals of its subjects are taken to extremes.

Transcendence has been unfairly savaged by critics no doubt expect something more profound or intelligent, but on its own merits Transcendence thrives. It's ridiculous, it's fun and it's interesting.

Having been directed by an Oscar-winning cinematographer it's gorgeously shot as well, and Mychael Danna's score builds the tension expertly – giving the film's early stages an unnerving feel as scientists push the moral boundaries of nature.

The subject matter coupled with the big budget and wealth of quality actors makes Pfister's debut ambitious in the extreme, and at the very least he deserves credit for that. Transcendence certainly has problems, but as a film about putting Depp's brain inside a computer I found it exactly as barmy and fun as that sounds.