According to TvGuide, Beals will play "Sophia Conrad, a smart, commanding and sexy CIA operative. Sophia joins Castle and Beckett in the search for a fellow CIA agent who has gone rogue and is now the key suspect in Castle and Beckett's murder case. But Beckett soon becomes distracted by the fact that Castle and Sophia have quite a complicated history."
For those unfamiliar, Castle is a drama series in which murder-mystery writer Richard Castle, played by Nathan Fillion, joins forces with Detective Kate Beckett, played by Stana Katic, to solve murders in New York.
New episodes of the series start January 9 and Beals' two-parter is slated for February.