British intelligence officer George Woodhouse is tasked with investigating a leak of a top-secret software, Severus, and is given one week by his superior, Meacham. One of five suspects is his wife, Kathryn, also an intelligence officer. At dinner with the other four suspects—satellite specialist Clarissa, her boyfriend Freddie, psychiatrist Zoe, and Zoe’s boyfriend James—George drugs their food and exposes Freddie's infidelity, causing tension.

When Meacham dies of a suspicious heart attack, George becomes increasingly suspicious of Kathryn, especially after discovering a film ticket stub in her trash and learning she’ll be traveling to Zurich. He uses a redirected spy satellite to watch Kathryn meet with a Russian operative in Switzerland and discovers she has access to a bank account with ₤7 million. Meanwhile, Zoe questions Kathryn's priorities during a psychiatric session, and Freddie informs Kathryn of George’s suspicions.

It is revealed that, while George watched Kathryn in Zurich, a Russian operative escaped with the Severus program, heading to Eastern Europe to cause a nuclear meltdown. Kathryn suspects their boss, Stieglitz, intentionally leaked Severus to destabilize Putin's government. She leaks the operative’s location to the CIA, resulting in a drone strike killing the Russians. George polygraphs the suspects to uncover when they first learned of Severus, and he and Kathryn realize they are being set up.

At a second dinner, Kathryn places a gun on the table, and George reveals multiple secrets about the suspects’ involvement. James confesses his role in the plot and attempts to shoot George, but Kathryn kills him. The colleagues dispose of James' body, and Kathryn informs Stieglitz that his plot failed. She and George reaffirm their love, realizing the ₤7 million is still available for them.