Given that any adult who is happy to ride a shopping trolley deserves to be an object of pity, Julien (Canet) is a pretty sad sack.

His life - from primary school to 30 something engineer - has been consumed by the bizarre relationship he has enjoyed with childhood friend Sophie (Cotillard).

Nothing - not his marriage to a dutiful if dull wife nor hers to a footballing superstar - has impinged on the personal arrangement.

As children, Sophie dared Julien to release the handbrake of the schoolbus…which he did, setting in train a lifetime of alternate challenges.

These hinge on the custody of a tin box in the shape of a merry-go-round: whover is in possession has to act out the dare.

These started off innocently enough - spilling ink, petty insolence - but soon give way in adolescence to sexual humiliation and into Dangerous Liaisons territory.

The problem is that as soon as the two lovers cross over into adulthood they cease to be inventively naughty kids but tediously delinquent grown-ups.

Dark romantic fantasy of this hue needs to have sympathetic characters, but Julien and Sophie fall into the "slap them they're French" bracket.

There are attempts to buy affection - Julien's mother's lingering death and Sophie's status as a persecuted Polish immigrant - but they never emerge as people you'd want to spend any time with.

One point of interest is each other's nervous suspicion that the dare system they adhere to means declarations of love will be treated as a sort of double bluff.

However, the fact that your loyalties end up lying with their respective partners spells doom for any hopes that you might cheer them on in the ultimate challenge.

Tim Evans