There is a strong logic for bringing the BBC's popular spy drama Spooks to the big screen.

The show, which ended in 2011 after 10 series, was extremely popular and sported production values often more muscular than its medium required. And who wouldn't want to see Peter Firth's tortured MI5 boss Harry Pearce strut his stuff for a wider audience?

And yet, while it's perfectly watchable, one can't help feeling disappointed. The Greater Good feels like a prize fighter finally getting a crack at a world title and retreating into his shell.

It appears to be business as usual for the British secret service: a high-profile assignment, the CIA looming over its shoulder, egg on face. It seems obvious that if you're transporting a captured jihadi terrorist, you don't get caught in a traffic jam; our agents do exactly that, Adem Qasim (Elyes Gabel) escapes, and Sir Harry is the fall guy.

But there's almost never a cock-up without a mole. And Pearce goes rogue to find out who that is, reaching out to former agent Will Holloway (Kit Harington) to help him. We're immediately offered a trio of potential candidates, all at the top of the tree: Harry's MI5 boss (Tim McInnerny), his deputy (Jennifer Ehle) and the politician overseeing the secret services (David Harewood).

With McInnerny oozing malevolence, Harewood familiar for his oleaginous and self-serving CIA chief in Homeland, and Ehle deliciously delivering most of the best lines as though she's too good to be true (to hot-headed Will: "Unless you're actually going to murder him, I think you've made your point"), each of these makes an eligible candidate. The question of which it is keeps us guessing right to the end.

As well as the whodunit, the scriptwriters (veterans from the show) ensure two key characteristics of the Spooks brand make the transition: a cold-hearted lack of sentimentality for its principal characters, which means we never know which seemingly essential character could be ruthlessly despatched next; and a murky moral terrain, epitomised here by Pearce's greater desire to catch the mole than to stop the bomb threat to London. Pearce has always been a tough nut, but as a spy out in the cold he appears to be frozen to the core.

Clearly, then, there's material to enjoy here. It's particularly satisfying to see Firth stick to his guns as Harry, doing nothing to make the spy endearing or touchy-feely, as committed to glumness as Gary Oldman was to greyness as George Smiley in Tinker Tailor Soldier Spy.

The problem is with everything that takes place around Harry. Surprisingly, the action sequences merely pack the same punch as they did on television - which means they're underwhelming for cinema. A sequence in Berlin's Alexanderplatz comically illustrates the weakness: when Jason Bourne took to the famous square, the result was full of movement, excitement and tension; when Harry and Will come to town, they could be pitching up at any old bus stop in the UK.

Similarly, the younger characters who are a staple of the show, Harry's action heroes in the field, are here found wanting. Harington might scowl to good effect in Game Of Thrones, but both he and Tuppence Middleton as an ambitious junior spy are fetching, competent and completely lacking in dramatic heft.

Spooks falls between the action thrills of Bond and the cerebral pleasures of Tinker Tailor. I'd be open to a sequel, just to see if Harry can get any more miserable, though I'm not sure that's reason enough.