Dirty Business review – if this doesn’t incite righteous anger over our filthy water then nothing will

We know, because ITV’s Mr Bates vs the Post Office showed us, that television drama can suddenly intensify public disgust at a scandal, forcing official attitudes to change. Will Dirty Business, Joseph Bullman’s drama-documentary on the great English and Welsh water pollution shame – whose storylines are based on real-life events – be another TV show that moves the needle? If this doesn’t do it, perhaps nothing will: this is a fist in the face, a blast of controlled fury that mounts an unanswerable case for the prosecution.

The Cotswolds, 2016. Two neighbours, recently retired and hungry for a project, notice brown murk in the previously beautiful River Windrush. By profession, Ashley Smith (David Thewlis) was a real-life “Line of Duty” cop investigating corrupt cops, while Peter Hammond (Jason Watkins) was an Oxford maths professor. Together they look into a curious dumping of sewage and, when the explanation given by the privatised local water company doesn’t add up, they dig in. Ash’s infallible nose for dishonesty, married with the algorithm Peter devises to find patterns in confusing data, builds a picture of water infrastructure destroyed by three decades of underinvestment, leading to environmental calamity on a staggering scale across the country, with thousands of instances of rivers and seas tainted by untreated sewage. Real footage, shot by campaigners to show the extent of the damage, is woven into the drama.

A second timeline starts in 1999, when Mark and Julie Preen (Tom McKay and Posy Sterling) take their two daughters on holiday to Dawlish in Devon, which Julie has chosen because it has Blue Flag status, indicating a clean beach. But they find what seems to be effluent pumping out of a pipe on the shore. Eight-year-old Heather steps in the dirty water. Within two weeks, she has died from E coli O157 poisoning.

Ultimately, the cause of the outbreak was not identified, and a verdict of misadventure was returned by a jury. The coroner’s recommendations included the tertiary treatment of all sewage in the area to make it pathogen-free, as well as a summertime ban on dogs on the beach.

A reconstruction of the Preen family’s day at the beach … with Posy Sterling (left) as Julie and Tom McKay (right) as Mark Photograph: Rob Baker Ashton/Channel 4

Bullman navigates what could have been an awkward tonal clash. While the scenes in 2016 capture the lovely faux-mocking banter between garrulous Ash and nervy Peter, and are often as funny as they are disquieting, the events of 1999 onwards are pure horror for the Preens, their true story ending with a further tragedy that is drawn here with a devastating starkness.

It works, perhaps, because Dirty Business is so good at using comedy as a weapon. This starts with the corporate statements Ash and Peter receive, supercilious evasions that are read by the actors playing the executives in exactly that tone, direct to camera. When the two men realise the problem is as much with the regulator, the Environment Agency, as it is with the water companies, a third story strand begins, in the EA offices in 2008, and the dark absurdity intensifies.

A change is announced towards the end of the Labour administration, the effects of which are to be greatly worsened by David Cameron’s drive to cut spending and slash regulation in the 2010s: “operational self-monitoring” moves the burden of identifying potential breaches of environmental law from the EA to … the water companies themselves. The scene where this news is broken to incredulous EA staff by a manager (played by Charlotte Ritchie) plays out like a comic sketch about a boss informing workers of a manifestly ridiculous new company policy.

Bullman hits a lot of ancillary targets: the uselessness of fines as a punishment for organisations that make larger sums by breaking the rules; the revolving door of employment between regulators and the companies they regulate; the inflated salaries paid to water bosses. But, from his choice of how to begin episode one onwards – he opens with newsreel of Margaret Thatcher in 1989, predicting that water privatisation “will go very successfully indeed” – he never loses focus on the fundamental point that it is highly unusual for a country to hand water provision over to private companies who exist to make money for investors, and that in the absence of a government that is minded to undo privatisation, not much will change. A right-of-reply statement provided to the programme-makers by the EA stresses that operational self-monitoring is to be discontinued by the government, but if that gives you hope, Dirty Business has well-chosen clips of Keir Starmer and the former environment secretary Steve Reed that will make you sink again.

Unlike some similar true-life dramas where the plucky underdogs inevitably triumph, Dirty Business knows the fight is not won. Watkins brilliantly embodies the stress of being a citizen battling opponents who have a vast advantage in money and information: towards the end of the series, Peter cracks and nearly gives up. “It’s exhausting,” he says to Ash, anticipating that their latest revelation will be met with another round of condescending obfuscation and “investigations” that don’t go anywhere. “Nothing will happen.”

Is he right? That’s up to us now, because TV has done its bit. Dirty Business could not be clearer.

• Dirty Business is on Channel 4