"Castle" fans waited four years for Castle and Beckett to realize their love for one another and another three to see them finally exchange wedding vows. But in one fell swoop, their marriage came crashing down on Monday night.

In part two of the eighth season premiere, Kate (Stana Katic) packed a bag and said goodbye to Rick (Nathan Fillion) before walking out on him. She pleaded with him to trust her and to find it in his heart to forgive her when she finished what she needed to do alone.

What he didn't know was that she had made the choice to find the powerful individual behind LOCKSAT and working the case together could put him in mortal danger. The investigation has sparked a fire in her not felt since solving her mother's murder and it's come back with a vengeance.

"Last season, Beckett tried to be the happy wife and great detective, but what the case that launches this season did is make her realize that there's something a little "broken" inside of her," co-showrunner Terence Paul Winter told TVLine. "Hopefully by solving this, she will come face to face with what she needs to get out of it."

Despite his wife's abrupt departure, Castle isn't giving up on his marriage and will remain optimistic in his journey to win her back.

"The way he's approaching it is: "No matter what, I'm going to win her back. She needs me," co-showrunner Alexi Hawley told TVLine. "So he puts a very positive spin on it ... rather than get too heavy with his suspicions or fears."

He also stays determined to be involved in her other murder investigations. Winter promises the two will find a way to continue working and building theory together.

"They have all those delightful put-togethers. The sparks between them are electric if not more than ever, and that's one of the main reasons why we made this bold choice," Winter said.

Even when Beckett tells him he's not joining her case, Castle weasels his way in by going undercover as a college professor in next week's episode.

"I want my wife back," Castle says in the promo. "Working together is why she fell in love with me."

Unlike previous long-term cases on "Castle," the show will follow Beckett's investigation into LOCKSAT over the next five episodes in addition to the standalone cases with everything culminating in the fall finale.

"Castle" airs on Mondays at 10 p.m. on ABC.