The Diplomat Season 4 Trailer Puts Kate Wyler on a Collision Course With Grace and Hal — as a Secret Russian Weapon Threatens the US-UK Alliance

The Diplomat is heading into its fourth season with Kate Wyler facing an extraordinary problem: her own husband and the president may have helped steal one of Russia’s most dangerous weapons.

Netflix’s new trailer picks up after the Season 3 cliffhanger and shows Kate, played by Keri Russell, confronting the increasingly risky decisions being made by President Grace Penn, played by Allison Janney, and Vice President Hal Wyler, played by Rufus Sewell.

With the fragile relationship between the United States and Britain under severe pressure, the fallout threatens to become much bigger than another disagreement inside the White House.

And Kate is making it clear that she believes a line has been crossed.

Kate confronts Grace over the risks

The trailer shows Kate challenging Grace directly over the administration’s approach.

“Your tolerance for risk to human life seems higher than it was,” Kate tells the president.

Her concern becomes even clearer during an exchange with White House Chief of Staff Billie Appiah, played by Nana Mensah.

Kate warns that a covert military incursion has taken place on an ally’s territory and describes the action as “over the line.”

Billie’s response is brief: “The line moves.”

Those exchanges point to the central crisis awaiting Kate when the series returns.

Grace and Hal’s secret could push the US toward war

Season 3 ended with Kate suspecting that Hal and Grace had double-crossed her.

At the center of that suspicion is Poseidon, described within the series as a Russian nuclear weapon capable of devastating destruction.

Kate fears Grace and Hal may have taken the weapon, an action that British Prime Minister Nicol Trowbridge, played by Rory Kinnear, could regard as an act of war.

Netflix’s Season 4 preview confirms that their conspiracy to steal Russia’s most powerful weapon could push the United States toward war while damaging the already fragile relationship Kate has worked to maintain between Washington and London.

Todd Penn becomes part of Kate’s problem

The crisis isn’t confined to Grace, Hal and Kate.

First Gentleman Todd Penn, played by Bradley Whitford, becomes increasingly involved and begins encroaching on Kate’s professional territory.

But the shifting relationships between the four central characters are even more complicated.

Creator, showrunner and executive producer Debora Cahn said Season 4 is built around Kate, Hal, Grace and Todd — two married couples whose public responsibilities are increasingly interfering with their private relationships.

Cahn described them as two couples trapped together on a deteriorating roller coaster.

Grace and Hal develop a closer bond, while that relationship pushes Kate and Todd into an uneasy alliance of their own.

Two marriages collide with an international crisis

The changing alliances create trouble on several levels.

Kate and Hal’s marriage was already under considerable strain, although the end of Season 3 briefly suggested they might be moving toward a better place.

That hope is complicated by Kate’s growing suspicion that Hal has betrayed her alongside Grace.

Todd, meanwhile, has noticed the closeness developing between his wife and Hal.

According to Netflix’s description of the new season, Kate and Todd eventually find themselves working together as they attempt to stop the Grace-Hal alliance from disrupting the balance of power both internationally and at home.

Debora Cahn says viewers have reason to worry

Cahn has made clear that the escalating power struggle will carry serious consequences.

She described working with Russell, Sewell, Janney and Whitford as an “insane bag of candy” and said she was excited about what the fourth season would bring.

But her comments about the story itself were considerably more ominous.

“There’s plenty to worry about,” Cahn said, adding that watching who gains power — and seeing the possibility of that power being badly misused — can become “pretty chilling.”

The Diplomat returns in October

Allison Janney and Bradley Whitford have been promoted to series regulars for Season 4, as has Nana Mensah. Keri Russell and Rufus Sewell return as Kate and Hal, with Ato Essandoh and Ali Ahn also among the returning cast.

The first three seasons are already available to stream on Netflix.

Season 4 arrives on October 15, with Kate caught between a deteriorating marriage, an increasingly volatile international relationship and a conspiracy involving two of the most powerful people in the US government.

And if the new trailer is any indication, the political crisis and the personal betrayals are about to become impossible to separate.