Castle Season 6 premiered Monday (Sept. 23) with an episode called, "Valkyrie." The fans are left worried over a huge cliffhanger. Why this big problem so soon for Rick (Nathan Fillion) and Kate (Stana Katic)? Meanwhile, Lisa Edelstein, who plays Rachel McCord, is proving herself gracefully to Cas-kett fans.

Spoiler Alert: This review contains spoilers for those who have not seen Castle Season 6, Episode 1 ("Valkyrie"). Read at your own risk.

The ABC procedural dramedy opens to where the series left off in Castle Season 5 - at the landmark swings, in the middle of Rick's proposal. Katherine Houghton Beckett says yes to Richard Edgar Alexander Rodgers Castle! All is well from there until - Castle sinks his teeth into a classified FBI case.

Rick and Kate are such mature and reasonable characters in Season 6. No one's hiding or running away. No one's being too insecure. Both are explicitly on board the whole marriage idea. But just when there's no more conflict between the two - the criminals come in to mess up everything.

As mentioned in spoilers, viewers meet Myko Olivier as Pi in Castle Season 6 Episode 1. Alexis (Molly Quinn) kind of takes him home as a souvenir from her Costa Rica trip. He's staying indefinitely at Castle's apartment. The old man is worried, but that will be the least of his concern by Episode 2.

After stepping into a case involving bad guys dealing with a chemical weapon, Castle has 24 hours to live. Kate delivers the news to Rick Castle in the last scene of the Season 6 premiere. "To be continued" is the last thing viewers want to hear after such a revelation. It is, however, safe to assume the titular character is not going to die. Still, Cas-kett fans are hoping this gets sorted out soon in the next episode.

Castle Season 6 Premiere: Stana Katic Gets What She Wants

Stana Katic has said in earlier interviews Beckett would be foolish if she passes up on a rare opportunity, all for romance. The stunning actress points out Rick Castle is a millionaire who can easily get on a plane. Distance won't be a problem between him and Kate. And it's exactly what viewers saw in Season 6, Episode 1.

In "Valkyrie," Rick makes a surprise trip to Washington D.C. when it becomes clear Kate would not make it to New York for the weekend. But they both later conclude it's not that much of a good idea.

Seeing Rick and Kate deal with a major problem so early into Castle Season 6 is a good thing. One, it shows that there is still a lot of exciting stories to explore even after the couple's engagement. Two, the external conflict is a chance for the two characters how strong they've become, internally, through to their very cores.

"The proposal is a big step, but we also wanted to challenge the relationship right away," Castle creator Andrew W. Marlowe told TVLine.

Other reports indicate Rachel McCord will be helping Kate decide on some wedding details. Check out the slideshow to see some Tumblr fandom posts on the sixth season premiere of Castle.

Castle Season 6, Episode 2 airs next Monday (Sept. 30) on ABC. Lisa Edelstein is returning for two more episodes. Rick jokes to Kate that she's not invited to their wedding. But there's no telling whether McCord would die or not ahead of the big event. (Why just three episodes?)

More 'Castle Season 6' and TV Guide Features:

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NCIS Season 11 Spoilers: Cote de Pablo Exits, Michael Weatherly Teases 'Christmas Feel' Closure to Tiva Fans [PHOTOS, VIDEO]
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