Ian McKellen and director Bill Condon reteam once again for this adaptation of Nicholas Searle's potboiler The Good Liar. But where their previous collaborations - James Whale biopic Gods and Monsters and Sherlock Holmes fan-fiction Mr Holmes - saw McKellen play men battling demons and repressed desires, here McKellen gets to play an out and out wrong 'un with no such self-reflection, nor even the faintest hint of a moral compass.

McKellen is wrinkly rotter Roy Courtnay, an octogenarian con artist who sets his sights on the life savings of widow Betty McLeish (Helen Mirren), which total over £2 million. Faking a dodgy hip, Roy is invited to move into Betty's suburban home, out beyond the greenbelt of London (a striking overhead shot of the estate gives it the appearance of a crop circle planted in the middle of nowhere), if not her bed. The two form a bond somewhere between platonic and romantic, which Betty labels "companionship", enjoying trips into London to see Inglorious Basterds (which prompts an amusing post-screening discussion regarding its historical accuracy) and sample Turkish cuisine.

Roy's plan is to convince Betty to set up a joint account and then siphon her half of the funds from the fake account. He seems to be getting close to bringing her around to the idea, but her pesky grandson (Russell Tovey) has marked Roy as a bad egg from the off and keeps poking his nose into his business. There's also a previous victim of one of Roy's cons who may be out for revenge.

If you can't figure out where this is all headed within the first 10 minutes, please send me your bank account details and I will promptly deposit the sum of $1 million into your account, honestly. The Good Liar is as predictable a thriller as you'll see all year, yet it still struggles to spin its yarn in a convincing manner. Too much of its plot is driven by improbable coincidences, like one character relying on another to do something that there's no real guarantee of them actually doing. Elsewhere characters behave in a manner that makes no sense when their true motivations are later revealed.

As a thriller, it's a mess, and if you're only interested in seeing The Good Liar for its twisty plot, you'll be highly frustrated with how insulting it is to the audience's intelligence. It manages to get by for its first half, until a diversion to Berlin, which brings in wartime flashbacks that plunge the film into a pit of silliness it never manages to claw its way back out of. The finale involves a revelation that is so dark it completely jars with the knockabout caper tone the movie had adopted up to that point.

As nonsensical as The Good Liar is, it's undeniably a lot of fun watching McKellen and Mirren lend some class to the proceedings. McKellen has always had two sides to his screen persona, the affable English gent and the malevolent villain, and the role of Roy allows him to exploit both. It's a thrill watching him switch from the doddery old codger who claims to be happy with Betty's companionship to the foul-mouthed mobster we see whenever she's not around. At a time when actors over a certain age are too often saddled with stately but dull roles, it's a blast watching someone like McKellen get to revel in "he's behind you" panto villainy. I can't think of many younger actors who could save The Good Liar from itself in the way McKellen and Mirren have with their worldly charms.

The Good Liar - 3 Stars out of 5

Directed by: Bill Condon

Starring: Helen Mirren, Ian McKellen, Jóhannes Haukur Jóhannesson, Jim Carter, Russell Tovey