“The OA” creators Brit Marling and Zal Batmanglij have set a new limited series at FX, the network announced Friday.The project, which will star Marling in a “key role,” centers on an amateur sleuth as she attempts to solve a murder at a secluded retreat. Marling and Batmanglij will both direct.The series centers on “gen Z amateur sleuth” Darby Hart, who is “invited by a reclusive billionaire to participate in a Retreat at a remote and dazzling location,” per FX’s series description.