Castle and Beckett's Relationship Is in Jeopardy on 'Castle' Finale [Exclusive Video]

A lovers' quarrel and an uncovered secret might drive Richard Castle (Nathan Fillion) and Det. Kate Beckett (Stana Katic) apart in the "Castle" Season 5 finale, "Watershed." In this exclusive sneak peek at Monday's episode, Castle finds a used boarding pass of Kate's, and he's downright angry, confused, and hurt over what it might mean.

[Related: 'Castle' Creator Teases an 'Emotional Cliffhanger' of a Season Finale]

Beckett confesses that she flew to Washington, D.C., to interview for a federal task force position. "I'm sorry, you interviewed for another job in another city, and you didn't tell me?" asks Castle, shocked. Beckett insists that she didn't tell him because it was only one interview and she knew the news would upset him, which it clearly does -- but only because she hid it from him, Castle claims.

[Related: The 'Castle' Cast Reveals Their Favorite Episodes So Far]

"Castle, this isn't about you. This is about me. This is about my life," she says, before admitting that she is seriously considering this opportunity. When Castle suggests that it would mean a future without him, Beckett doesn't deny it. She only says, "Please don't do this. Please don't make this about us." But as Castle sees it, the move would mean the end of their relationship.

Will Beckett actually leave Castle behind to further her career? Find out on the "Castle" season finale Monday, 5/13 at 10 PM on ABC.