Stana Katic and Nathan Fillion

Love might not be in the air for Castle (Nathan Fillion) and Beckett (Stana Katic) after all.

The first promo for Castle's fifth season shows the pair enjoying a few moments in post-coital bliss — but not for long.

The voiceover echoes most fans' responses to the Season 4 finale, describing the long-awaited hookup as "the best night of their lives." But it also warns that when the ABC drama returns, "they'll find out if it was the biggest mistake they ever made."

Prison Break's Jodi Lyn O'Keefe to stir things up on Castle

Watch the full promo below. Do you hope Castle and Beckett stay together?

Castle returns Monday, Sept. 24 at 10/9c on ABC.