Kenneth Branagh proves a born farceur in Sean Foley’s adaptation of Francis Veber’s classic French farce “The Painkiller.” As an anonymous hitman who holes up in a hotel room only to find himself caught up with the suicide next door, Branagh comes brilliantly, hopelessly, beautifully unstuck. All the poise that he brings to the stage makes it all the funnier when he loses control and jabbers away, drugged, concussed and manhandled. But Branagh’s a fine student of theater too and, under Foley’s skilful direction, the real joy is seeing classic routines and set pieces, door slams and jelly legs, buffed up to their very best.

It says a lot about British theater’s snooty disdain for French theater that Foley’s West End adaptation of Verber’s boulevard comedy, first seen at the Lyric Belfast five years ago, is the play’s first outing in English. In France, “Le Contrat” has been a rep staple since 1969 and, on this evidence, it’s not hard to see why.

There’s an elegance to this farce. The best follow their situation through with infallible logic, so that no matter how improbably events unfold, they’re always still just about credible. So it is here. It makes perfect sense that a hired assassin would need to take care of the suicide next door before taking care of his target, and that all manner of concerned individuals — porters, doctors, ex-wives and police — would turn up to check in, no matter that the last thing a hitman needs is company.

The beauty of Veber’s set-up is in its oppositions — the clash of international mystery man and suburban mid-life crisis, the alpha male and the omega. Branagh’s assassin is refined, slick and level-headed. He whips a sniper rifle out of the air vent in a flash and, with every knock on the door, stashes it swiftly out of sight. His neighbor, meanwhile, is the maudlin, pudgy and lackluster Brian Dudley (Welsh comic Rob Brydon), a failed photographer for a local newspaper, pining for his ex-wife (Claudie Blakley) and so self-pitying that he never has the slightest of suspicions about his new best mate. Brydon’s always done a nice line in sad-sack bores, and Brian is the sort of irritant that ought to come with a hazard label.

So should hotel rooms. With their identical suites and interconnecting doors, they make the perfect seedbeds for farce, and Alice Power’s mirror image design — an imaginary wall down its center — becomes a key player in proceedings. Foley directs with a keen eye for the uncanny, so that the two men fall into step unwittingly, but he instills the freshness of clean sheets with contemporary gags about cushions, ketamine and porn. There’s strong support from Mark Hadfield as a camp, people-pleasing bellboy and nobody blows their top like Alex McQueen, who turns from puce to purple as a pompous psychiatrist.

Nonetheless, it’s Branagh that keeps the laughs coming, one on top of another. As an actor of such status and control, his every gesture practiced and finessed, it’s a delight to see him lose both. As the situation runs away with him, leaving his assassin semi-conscious and semi-clothed, everything deserts him. His speech slurs, his legs give way, his face seems to swell. Ironically, his collapse is controlled — sometimes noticeably so — and he zigzags around dizzily, dancing little dances and battling to regain composure.

In the end, it’s Brian who takes control, even if Foley can’t make the personality swap entirely organic, and, when Branagh’s killer reveals his own mundane past, Veber’s play suggests that we all need to give up on life if we’re to live to the full.