When both the UK and the US are advocating for military action in the Middle East, Simon Foster (Tom Hollander), the Minister for International Development and MP for Northampton, inadvertently declares during a BBC Radio 4 interview that a war is "unforeseeable." This statement provokes the ire of Malcolm Tucker (Peter Capaldi), the Prime Minister's Director of Communications, who harshly reprimands Simon and instructs him to adhere to the official line. In a bid to support Simon, Judy Molloy (Gina McKee), the Director of Communications for the UK Department for International Development, steps in, but even she falls prey to Tucker's fury as he storms out.
As Simon flounders, his new aide, Toby Wright (Chris Addison), with the help of his girlfriend Suzy (Olivia Poulet), manages to get Simon to attend an important Foreign Office meeting. This gathering, orchestrated by Michael Rodgers (James Smith), Director of Diplomacy, features Karen Clark (Mimi Kennedy), the US Assistant Secretary of State for Diplomacy, who is against military intervention. During the meeting, she raises concerns about a report authored by her assistant, Liza Weld (Anna Chlumsky), titled "Post-War Planning, Parameters, Implications and Possibilities" (PWPPIP), which strongly argues against the intervention due to insufficient intelligence sourced from an unverifiable informant known as "Iceman."
Amidst rising tension, Simon is ambushed by reporters and, attempting to appease them, contradicts himself by stating that the government must be ready to "climb the mountain of conflict," incurring another rebuke from Tucker. Meanwhile, back in the US, Karen and Liza suspect that Linton Barwick (David Rasche), the US Assistant Secretary of State for Policy, has quietly initiated a secretive war committee. As the plot thickens, Bob Adriano (Enzo Cilenti), an advisor to Linton, informs him that Karen might be onto the clandestine operations. In an effort to manipulate public perceptions, Linton alters meeting minutes to reflect only pro-war sentiments, obscuring any mention of the war committee.
In a twist of fate, Tucker sends Simon on a mission to the US for fact-finding, a decision that leads to Simon dismissing Judy from his team. At a networking event, Karen partners with Lieutenant General George Miller (James Gandolfini), who also opposes military action, and suggests that Simon's involvement in the conflict discussions could provide a platform for dissent. During their time in Washington, Simon and Toby receive an invitation from Karen to attend a Future Planning committee meeting, but Toby inadvertently shares the details with a CNN contact, initiating a series of unforeseen complications that culminate in a rendezvous with Liza.
Tucker soon arrives in the US, only to discover that Linton has cleverly sidelined him while holding the real war committee meeting at another venue. Due to Toby's leak, the Future Planning committee faces overwhelming media scrutiny, yet neither Karen nor George manages to obtain substantial information about the impending war. Both turn to Simon, seeking his endorsement for their differing agendas, but he remains noncommittal, struggling to articulate any significant stance.
As tensions flare back in Northampton, Simon's constituent, Paul Michaelson (Steve Coogan), confronts him regarding a dilapidated office wall threatening to cave into his mother's garden, prompting negative press coverage around Simon's perceived negligence. In the face of mounting pressure, Karen demands a scapegoat for the leak about the war committee, leading Simon to suspect that Judy might be the culprit. The turmoil continues when Suzy finds out about Toby's brief affair with Liza, leading to their breakup and a dramatic fallout.
In a rush to expedite the security council vote on military intervention, the President pressures the situation by vetoing tariffs on Chinese imports, making it clear that he wishes for China to abstain. In a final gambit, Simon instructs Judy to leak that he plans to resign if a pro-war vote occurs. Together with Judy, Toby, and Tucker, he heads to New York City for the UN Security Council vote. However, Tucker learns of the leaked PWPPIP report, which threatens to derail the vote.
Ultimately, it comes to light that the leak stemmed from Michael Rodgers, who unwittingly disclosed sensitive information. As the situation spirals, Tucker, aided by Senior Press Officer Jamie McDonald (Paul Higgins), seizes control by concocting details from the report to present a biased narrative favoring intervention. With this manufactured intelligence at hand, the committee endorses military action.
In the aftermath, George informs Karen of his decision not to resign now that the war is underway. Meanwhile, Simon confronts the reality of his inevitable resignation, but Tucker preemptively dismisses him from the cabinet before Simon can act on his intentions. The chaotic political machinations ultimately lead to the appointment of a new Minister for International Development, signaling the end of Simon's tumultuous journey.
