In this, my new favorite episode of Castle, we meet his ex-wife Meredith (Darby Stanchfield). I thought she might be the grounded one, passing that demeanor on to Alexis. Instead, she's an actress, with whom Castle has great bedroom chemistry, but whose whimsy makes him seem positively responsible.
After a night of dalliance with Meredith, Castle helps Beckett find out why a woman was murdered. Leverage's Aldis Hodge played one of the suspects, pulling off a convincing West African accent.
At the climax, Castle joins Beckett and her fellow officers on a raid, worrying me that he might be implausibly heroic. Before he goes too far, though, he gets a cell phone call and has to step aside to answer it. Brilliance.
Later, in a standoff with the armed culprit, Castle is heroic, but in an improvised, believable way.
I wondered where I'd seen Darby Stanchfield before. Checking IMDb, she played Shannon Gibbs in the NCIS episodes "Hiatus" and "Requiem", but in this season's "Heartland", a younger Shannon was played by Aviva Farber. I also discovered that in "Heartland", Mark Harmon's son Sean played the young Leroy Jethro Gibbs.