The Sentinel (12A) <!-- none onestar twostar threestar fourstar fivestar -->

Is it a sign that Michael Douglas has moved up or down in the world when, having been the leader of the free world - in The American President (1995) - he now plays, in The Sentinel, a veteran secret service agent who guards aUS leader's wife? To go from Caesar to playing the spear-carrier would seem to be a demotion, but, in fact, it's not the career slump it might have been. In the one role, he was allowed to show off his statesmanship and gravitas; in the other, he gets to prove his action-man credentials as one of the elite praetorian guard at the White House. He still has the camera trained on that sly and increasingly saurian phiz of his, and can just about hold the centre of a movie.

All the same, Douglas is in his sixties now, and the sight of him outrunning a much younger man is just one instance of The Sentinel stretching credibility a shade too tight. As Pete Garrison - how's that for a tough-guy name? - he plays a super-dedicated agent who once took a bullet for a president and yet got passed over when the plum jobs were handed out. (Could it be because the president he took the bullet for was Ronald Reagan?) Now he faces another crisis, after a conspiracy to murder the current president comes to light. What's more, the finger of suspicion points at a White House insider, and the smart money is on Pete.

The twist is that Pete actually is undermining presidential security, though not in a way anyone would guess. It transpires that he's not only protecting the First Lady (Kim Basinger), he's also been having an affair with her - yes, he's cuckolding the prez in his very own house! If that reflects badly on the commander-in-chief's powers of observation, it adds up to criminal negligence on the part of Pete's colleagues. They might have noticed, you'd think, how much time he spends mooching around the First Lady's chambers. Not even the top secret service agent and ex-best friend Dave Breckinridge (Kiefer Sutherland) suspects, and he has more reason than most: he's still smarting from the time Pete had a fling with his wife. Quite a swordsman, our hero - and with foxy young agent Eva Longoria just recruited to the team you wouldn't put it past him to start offering her some secret service, too.

The director, Clark Johnson, working from a script by George Nolfi, emphasises the agency's reliance on code-names and protocol, which always come garnished, in a movie like this, with a side-order of pomposity. I'm sure the secret service does excellent work, but it's a narrow American habit to regard the defence of their president as the most sacred job in the world. One can almost feel the film-makers swooning over the fetishistic apparatus involved in its execution: the gleaming black town-cars, the suits and shades, the ear-pieces, the procedural details of fingerprinting and phone-tapping. Their efforts to impress us might have been better adapted to devising a roadworthy plot. As it is, this can hardly help invoking the ghosts of earlier, better movies: the agent's race to foil a presidential assassination is a straight lift from Clint Eastwood and In the Line of Fire (and lacks the frisson of John Malkovich's unreachable villainy); Douglas's entrapment within the White House echoes Kevin Costner similarly entangled in No Way Out; and his adultery with Basinger might even be compared to that ill-advised fling with Glenn Close in Fatal Attraction - he's just playing with a different kind of fire.

The subject of homeland security holds a lot of heat, post-September-11, and The Sentinel makes a point of reminding us how the US president's life, and thus America itself, is shadowed by the direst emergencies. A montage of scribbled death-threats keeps flashing across the screen, and not of all them are in a foreign language. But if the necessity of vigilance is the film's message then it rings pretty hollow: when one of his trustiest aides is diddling the First Lady, what kind of protection can the President hope for against his deadliest enemies? Office romance can be tricky, but an Oval Office romance is asking for trouble.