Called "B", the movie is billed by its director -- known for several popular TV shows -- as "a work of democratic health" in a nation rattled by corruption scandals that have touched the left, the right and even the royal family.
Several thousand viewers have seen it since it opened last month but many more will have the chance, right at home, from mid-November when the film will be streamed over Internet for free.
It is "almost a documentary because the text is real", said director David Ilundain, 40, who financed the project through crowdfunding and filmed it in six days in a real courthouse.
"One hundred percent of what is said in its 78 minutes" was taken from a transcript of five hours of questioning of Luis Barcenas at Spain's High Court on July 15, 2013, he told AFP.
The long-running graft probe has been a source of great embarrassment to the conservative Popular Party, which took power four years ago. And "B" is a disturbing reminder as the party struggles in opinion polls
ahead of the December 20th poll.
Up until the day he was questioned in court, Barcenas -- the Popular Party's treasurer from 1988 until he stepped down in 2009 -- had denied running a slush fund that filtered cash donations from business magnates to party leaders.
The film shows police removing handcuffs from Barcenas, a former senator played by actor Pedro Casablanc, so he can put on a tie brought by his lawyer then face the judge's questions.
Run down by a stint in jail that he fought hard to avoid, Barcenas suddenly decides to provide the judge with a pile of documents -- the secret or "B" books -- he says proves the slush fund existed.
The former treasurer then names party officials who got money and whose initials he jotted down in these handwritten ledgers, which also listed the names of firms that made thousands of euros in donations to the party. Most were companies that won lucrative public works contracts.
One senator collected 700,000 euros ($794,000) as a reimbursement of her campaign expenses.
Barcenas tells the judge he personally gave top party officials, without any witnesses present, thousands of euros each month in 500-euro bills that came from the slush fund.
The former treasurer said Mariano Rajoy -- the president of the Popular Party since 2004 and prime minister since 2011 -- knew of the fund and received money from it until March 2010 when he got an envelope with €25,000.
But Barcenas spares former Popular Party prime minister Jose Maria Aznar, who served two terms from 1996 to 2004, telling the judge he "could not say" that the initials "J.M." in his ledgers corresponded to the former leader for money received during the 1990s.
High Court investigative judge Pablo Ruz and his successor Jose de la Mata concluded earlier this year that the Popular Party did indeed keep a secret set of accounting books that was fed by "exterior donations and contributions" for at least 18 years.
But they recalled that the hidden financing of political parties was not considered a crime, which explains why Rajoy has not been the target of a criminal investigation.
Along with five other defendants, Barcenas is waiting to go on trial for misappropriation, fraud and falsification of documents.
The movie ends with real footage of Rajoy addressing parliament over the affair in August 2013, saying he had made a "mistake" in "trusting" Barcenas.
Rajoy and other Popular Party leaders have denied ever receiving off the book payments.
Barcenas' court questioning has already inspired a play which was staged in theatres across Spain last year, and served as the basis for Ilundain's movie
"It is the first time that a film is based entirely on the statement given by a defendant" in Spain, said Joaquim Bosch, spokesman for Judges for Democracy, a leftist association of judges and magistrates.
"There were no voices which found this unacceptable," he added.
By Laurence Boutreux / AFP