Malcolm Reynolds and River Tam are together again, at least for one night.

TVLine reports Summer Glau will reunite with fellow "Firefly" veteran Nathan Fillion on an episode of the ABC drama "Castle." She'll play a private detective who goes head to head with Fillion's Castle.

It's the second reunion for the pair in recent months, as Glau recently appeared in an episode of "Con Man," the web series starring Alan Tudyk and Fillion. Fellow "Firefly" alums Adam Baldwin and Gina Torres have also appeared on "Castle."

Glau, whose credits include "The 4400," "Terminator: The Sarah Conner Chronicles" and "Dollhouse," also had a recurring role on "Arrow."