Illegal immigrant Okwe (Ejiofor) is touting for mini-cab business at the airport for "people let down by the system".

What we don't know that Okwe couldn't join the system if he wanted to - his alien status means he must exist on the margins beyond the reach of the authorities.

Struggling to earn a living as a cab driver by day and hotel receptionist by night, he shares a poky flat with Turkish asylum-seeker Senay (Tautou).

During one of these hotel shifts he makes a shocking discovery - a human heart clogging the toilet bowl in one of the rooms.

He can't tell the police, because of his immigrant status, so it is up to him to gradually unravel the mystery for himself.

What he discovers is a chilling "organs for sale" racket or, as he puts it, desperate immigrants "swapping their insides for a passport".

Frears is particularly strong on the fine detail of the invisible existence - the permanent glances over the shoulder for immigration - in this case a couple of goons with the menace and manners of the Gestapo.

Tautou's Senay is thankfully miles away from the whimsical Amelie while Lopez is evily convincing as the organ fixer and hotel kingpin, Sneaky.

However, it's Ejiofor that holds things together with a dignified performance through a state of affairs where the first thing to go is your self-worth.

His integrity is mortally compromised by his immigrant position but his character is tempered by a welcome dry sense of humour.

Things may get a little melodramatic towards the end - the "operating theatre" scene plays like a slapstick episode of Casualty - but Frears generally steers a steady course.

Ironically, the story was conceived by Steven Knight, one of the brains behind Who Wants to be A Millionaire?

Most of these sad victims would be happy with £10,000. Even if they had to forfeit a kidney to get it.

Tim Evans