Amid an international crisis, a career diplomat juggles her new high-profile job as ambassador to the UK and her turbulent marriage to a political star.

When a London bombing shatters her world, US diplomat Kate Wyler faces the ultimate test as her suspicions reach the top levels of the British government.

Ambassador Kate Wyler lives the particular nightmare that is getting what you want. She just accused Vice President Grace Penn of hatching a terrorist plot and admitted she's after the VP's job. But now the President is dead, Kate's husband Hal may have inadvertently killed him, and Grace Penn is leader of the free world. None of this slows Hal's campaign to land Kate the vice presidency. Kate steps into a role she never wanted, with a freedom she never expected, an increasingly complicated friendship with Foreign Secretary Austin Dennison, and an unnerving bond with First Gentleman Todd Penn.

In Season Four, two marriages threaten to eat each other alive. One catastrophic moment shatters the fragile peace Kate Wyler brokered between the US and UK, as well as the crumbling marriage she's trying to save. While she fights to stem the bleeding, President Grace Penn and VP Hal Wyler's conspiracy to steal Russia's most powerful weapon hurtles the United States toward war. Kate and Todd, at best an uneasy pair, unite to stop the Grace-Hal alliance from upending the balance of power abroad and at home.