Sent behind bars with a yawn and a sneer

“PRESUMED GUILTY”, a documentary film which was released in Mexico in February and banned two weeks later, is the opposite of a whodunit. From the beginning it is clear that Antonio Zúñiga, a mild-mannered computer-repairman, had nothing to do with the murder that took place near his Mexico City market stall in 2005. Forensic tests were negative; fellow stallholders gave him an alibi; the one supposed witness could not even describe his appearance. This quietly angry film, which has been shown at film festivals over the past couple of years, is a detective story in reverse, following Mr Zúñiga's difficult, dignified fight to overturn his conviction.

It sets out to shame Mexico's justice system, and succeeds twice. After the prosecution witness in Mr Zúñiga's trial made a dubious complaint that the film invaded his privacy, a judge suspended screenings on March 2nd. But the ban, seen as an attempt to suppress an embarrassment, has backfired gloriously. In the days before it kicked in, multiplexes were jam-packed. More than 1m tickets have been sold, making it by far the most successful documentary in Mexico's history. Pirate DVD hawkers stock it alongside Hollywood blockbusters.

It makes chilling viewing. In prison, Mr Zúñiga shares a cell with 20, sleeping in a space under a bunk bed known as the grave, his dreams disturbed by cockroaches. His girlfriend Eva, an embroiderer of mariachi costumes, brings him food, and is searched by guards who relish groping under her shirt.

The real creeps, though, emerge in court. Mr Zúñiga wins a retrial after discovering that his first lawyer had forged his documents. The case is heard by the same judge, who smirks and stifles yawns before handing down an identical guilty verdict and a 20-year prison sentence (there is no jury and, as the title suggests, the onus is on Mr Zúñiga to prove his innocence). The sullen, vacant prosecutor submits her closing arguments on a floppy disk; the bullying detective bristles with bravado but cannot look Mr Zúñiga in the eye. On the day of the final appeal, which quashed the conviction, Eva, whom Mr Zúñiga married and made pregnant while in prison, gives birth to their first child.

After a judge overturned the ban on the documentary, it is now once again on show in Mexico's multiplexes. Counter-appeals may follow. Throughout it all, DVD sales and hits on YouTube continue to mount up. Fortunately, Mexico's courts have proved as incapable of suppressing this film as they were of giving Mr Zúñiga a fair trial in the first place.