The film opens with a future (fictionalized) public inquiry in 2020, where a frustrated Dominic Cummings tries to explain to the commission that they do not understand how new technologies are changing politics and therefore society in the UK.
In 2015, Cummings rejected an offer from UKIP MP Douglas Carswell and political strategist Matthew Elliott to lead the Vote Leave campaign because of his disdain for “Westminster politics,” but agreed when Carswell promised him full control. Cummings is using “micro-targeting tools driven by an algorithmic database” distributed through social media and the Internet, instead of the traditional campaign of posters, phone calls and leaflets distributed by local MPs. Cummings rejects the approach of Nigel Farage and Arron Banks of Leave.EU to combine campaigns, as his data shows that Farage is an obstacle to a majority. Cummings’ technological approach is causing disagreements with Vote Leave MPs and funders. John Mills, the chairman of Vote Leave, tries to fire Cummings to merge with Leave.EU, but is fired instead.
Cummings and his Remain colleague, Craig Oliver, share their strategies and opinions of each other with their teams. Both identify the one-third of undecided British voters as a part of society that can be influenced. Oliver targets “jobs and the economy”, while Cummings believes that “loss of control” and Turkey’s possible accession to the EU will be of greater concern. Cummings refers to a strategy from Sun Tzu’s Art of War and avoids refuting his opponent’s slogan, instead focusing on his own message of “taking back control.” Cummings meets and hires Canadian Zach Massingham, co-founder of AggregateIQ, who proposes to create a database using social media tools of voters who are not on the British electoral register but are ready to vote to leave. Arron Banks meets Robert Mercer, who discusses the potential of social media database tools.
Cummings, using the AggregateIQ database, brings MP Douglas Carswell to Jaywick, a part of his constituency he didn’t even know existed, where a couple talks about losing their fortune. Oliver, using traditional focus groups, realizes that his campaign has failed to understand the concerns of many British voters, as members of one focus group fell out with each other and one participant broke down in tears: “I’m sick of feeling like I have nothing… like I have nothing! Like I don’t know anything. Like I’m nothing. I’m sick of it.” Oliver’s own staff becomes demoralized and angry.
In the final phase of the campaign, prominent Conservative MPs Michael Gove and Boris Johnson join the Vote Leave campaign, emphasizing the need to “take back control,” and Penny Mordaunt expresses concerns about Turkey’s accession on the BBC. Oliver hosts an emergency conference of the Tory-Labor Remain party with Prime Minister David Cameron and Peter Mandelson, but each side blames the other for the decline of the campaign. After the murder of Jo Cox MP, Cummings and Oliver have a drink and discuss the events. Cummings compares his campaign to having started a train that can’t be stopped, and Oliver replies: “Be careful what you wish for. You won’t be able to control it afterwards.”
On June 23, 2016, the United Kingdom voted to leave the EU by a narrow margin. After the victory speech, Cummings quietly leaves the Vote Leave campaign office. Then the film returns to the present during the (fictional) public inquiry of 2020, Cummings expresses his disappointment with the way the political system has reacted after the Vote Leave victory and walks out in disgust.