Before the film starts, director Mick Jackson gets up on stage and reminds us that, just a few days previously, the President of the United States sent his official White House press secretary in front of reporters to tell blatant lies about the size of the crowds at the inauguration. A day later, one of his closest advisers went on television to defend these lies as “alternative facts.”

“This,” Jackson says, “is a film about alternative facts.”

There really could not be a more eerily opportune time for the release of a film about Holocaust denial and the quest for the truth. Denial tells the true story of American academic Deborah Lipstadt, who is taken to court for defamation by Hitler apologist and “alternative historian” David Irving after she writes a book calling him Holocaust denier. The burden of proof is on Lipstadt to show that her comments disparaging Irving’s fake version of history were justified. In other words, she must prove that the Holocaust really happened, and that Irving purposefully distorted facts to try to argue otherwise.

It’s a riveting film, full of fiery dialogue and outstanding acting. (A particular shoutout should go to Rachel Weisz as Lipstadt and Tom Wilkinson as her tenacious barrister Richard Rampton, while Timothy Spall is grotesquely sinister as David Irving.) But in the current political climate, watching Denial is an unsettling experience. Through a barrage of legal jargon and rhetorical sparring, Lipstadt fights again and again to make her point heard: there is no such thing as an alternative point of view when it comes to whether or not the Holocaust happened. You can argue about why and how and if it could ever happen now, but the truth of its existence is a fact. Facts are not up for debate, and not all opinions are equal. If someone insists on repeating lies, call them a liar.

Denial is based on events that occurred in the late 1990s, back when Donald Trump was still buying beauty pageants and dominating the tabloid headlines only for his lascivious affairs and messy divorces. But his ascent into politics, right up into the White House, has laid the groundwork for a fresh generation of David Irvings. It’s not just about the number of people who turned up to watch an inauguration, although such a demonstrable lie about something so trivial is a chilling start to his presidency. We can barely keep up with the lies that tumble from Trump’s Twitter account and the mouths of his surrogates. Millions of illegal votes were cast in the election. Climate change is a hoax invented by China to hurt America. Trump never mocked a disabled reporter. Hillary Clinton started the vicious myth that Barack Obama was not born in the US. Trump was always opposed to the Iraq War. There is no evidence that Russia was responsible for the election cyber-attacks. Trump’s approval ratings would be much higher if the biased media weren’t waging an unfair war against him. His conflicts of interest have been satisfactorily mitigated by passing control of his businesses onto his sons. No one has more respect for women than Donald Trump.

The danger of repeating false claims such as these, without calling them what they are, is that it begins to blur the distinction between objective truth and differing opinions. As the trial progresses, Lipstadt grows incensed that Irving’s fake narrative (which includes mistranslations, factual errors, and clear disinformation) is being given equal weighting with the truth. She tries to argue that, for some things, presenting both sides of the debate when one side is clearly wrong is irresponsible. This is a lesson journalists reporting on Trump are beginning to learn. On Sunday, when Kellyanne Conway used her “alternative facts” line, NBC anchor Chuck Todd responded “alternative facts are not facts, they’re falsehoods”. The New York Times front page on Tuesday led with the headline “Trump Repeats an Election Lie”. CNN made a conscious decision not to live-broadcast Saturday’s White House press conference, but to report on it after analysing it for factual accuracy.

It should be highlighted that Denial is not a film about crushing free speech. The misconception crops up repeatedly that Lipstadt is suing Irving for being offensive. In fact the opposite is true. This was not a lawsuit about whether false historians should be allowed to publish their “research” (however painful it is to survivors) – in fact, the real Deborah Lipstadt has spoken out against banning Holocaust deniers from speaking. It was a lawsuit about the right of others to call such speech what it is: lies. In Denial, Irving is the special snowflake who cannot handle criticism and takes legal action over comments he finds offensive. That is another lesson for today’s world of post-truth politics and fake news – those who propagate it have a right to speak, and the rest of us have both a right and a duty to hold them to account. People like Richard Spencer, who has promoted articles arguing that the human civilisation does not need the “Black race” and musing on how it can be “disposed of”, are neo-Nazis, not “alt-right”. Trump’s claim that all Mexican immigrants are rapists is racist, not “politically incorrect”. And a those who peddle “alternative facts” are liars.

Deborah Lipstadt won her case, after a widely-publicised trial that sent shudders through the global Jewish community. But on the last day of court, there is a moment of doubt that raises questions about the very nature of truth. The judge asks whether it makes a difference that Irving might genuinely believe the falsehoods he is spouting. Can he justifiably be called a “liar”, if in his mind what he’s saying is true? The final verdict is that yes, he can, but it is clear from Irving’s reaction that he accepts neither accurate history nor the outcome of the lawsuit – he appears on television immediately afterwards arguing that, by his personal standards, he actually won. So what if President Trump’s mind is so warped by ego and narcissism that he really believes the photos showing low attendance at his inauguration were doctored by the biased media? How do we combat an administration that views truth as subjective, and how do we report on a man so convinced of his own alternative narrative that he will use the power of the White House to bend reality to protect it?

At the premier, Deborah Lipstadt introduced the film with both a warning and a call to arms. “There are facts, there are lies, and there are opinions,” she said. And it is our duty, in these turbulent times when truth is under attack, to be diligent about telling the difference.

Denial is released in UK cinemas on Friday 27th January.