Kate and Hal Wyler have never done anything halfway — especially marriage. When The Diplomat returns for Season 4 on October 15, their relationship will once again swing between emotional extremes while an even more dangerous crisis threatens the fragile peace between the United States and United Kingdom.

The newly released teaser shows Ambassador Kate Wyler (Keri Russell) attempting to repair her marriage to Vice President Hal Wyler (Rufus Sewell), even as suspicions surrounding Hal and President Grace Penn (Allison Janney) threaten to pull them apart.

As Kate herself puts it, she and Hal “do high highs and really low lows.” Season 4 appears ready to push those lows even further.

Kate Suspects Hal Has Double-Crossed Her

At the end of Season 3, Kate and Hal had finally come close to getting their troubled marriage back on track.

That progress quickly became complicated when Kate began to realize that her husband and President Penn may have double-crossed her.

Adding another layer of tension is First Gentleman Todd Penn (Bradley Whitford), who has noticed the growing closeness between Grace and Hal.

Creator, showrunner and executive producer Debora Cahn says those four characters form the foundation of the new season.

“This season is built on the foursome that is Kate, Hal, Grace, and Todd,” Cahn told Tudum.

She described the two married couples as being “strapped together in one car on a crumbling roller coaster,” with their powerful public responsibilities increasingly interfering with their private relationships.

Each of them occupies a demanding professional role, but those positions are making it increasingly difficult for them to function as spouses.

Grace and Hal’s Bond Creates an Unlikely New Alliance

The shifting relationships between the four characters will become even more complicated as Grace and Hal grow closer.

According to Cahn, their connection produces what she describes as a “change-partners” dynamic.

“The growing bond between Grace and Hal creates a not-so-functional alliance between Kate and Todd,” she explained.

That leaves Kate and Todd in an unusual position. Neither is necessarily comfortable working with the other, but circumstances may force them together as they confront what their spouses are doing.

The political and marital tensions therefore become increasingly intertwined, with the choices made inside the White House potentially affecting relationships on both sides.

Kate Isn’t Ready to Give Up on Hal

Despite everything working against them, Kate makes one thing clear in the teaser: she is still fighting for her marriage.

She tells Hal that she’s “not trying to get out of the marriage.”

Instead, she says, she’s “trying to get back in.”

That determination remains even as Cahn promises more heated confrontations between the pair.

The teaser also features Kate delivering her own distinctly Wyler-style version of a marriage vow.

“I, Katherine Wyler, take you for better or for worse, worse, and worse,” she says.

Whether Kate and Hal can survive another round of personal and professional betrayals remains uncertain, but their marriage is only one of the relationships facing a potentially devastating test.

A Catastrophic Event Threatens U.S.-U.K. Peace

Season 4 will also place the diplomatic achievement at the heart of Kate’s work in serious jeopardy.

A catastrophic moment shatters the fragile peace she has managed to broker between the United States and United Kingdom.

As Kate attempts to contain the fallout, an even larger threat emerges from Grace and Hal’s secret actions.

Their conspiracy to steal Russia’s most powerful weapon risks pushing the United States toward war.

With the international balance of power under threat, Kate and Todd form their uneasy partnership in an attempt to stop Grace and Hal before their alliance causes consequences both abroad and at home.

The result places the season’s two central marriages directly inside a geopolitical crisis, with personal loyalties and international responsibilities becoming increasingly difficult to separate.

Familiar Faces Return for Season 4

Keri Russell and Rufus Sewell return as Kate and Hal, with Allison Janney and Bradley Whitford reprising their roles as Grace and Todd Penn.

Ato Essandoh, Ali Ahn and Nana Mensah also return, while Janney, Whitford and Mensah have been promoted to series regulars.

Season 4 is executive produced by Debora Cahn, Janice Williams, Alex Graves, Russell, Peter Noah and Eli Attie.

With two marriages under extraordinary pressure, an uneasy new partnership forming and the United States potentially heading toward war, The Diplomat is setting up another volatile chapter for the Wylers and Penns.

The Diplomat Season 4 premieres October 15. The first three seasons are available to stream now.