The CW has released its official title and description for the first half of the two-part season three finale.
The episode is called “Perverse Instantiation – Part One” and here is the small summary:
PART ONE OF THE SEASON FINALE — Clarke (Eliza Taylor) finds hope in the most unlikely place. Meanwhile, Alie’s (guest star Erica Cerra) master plan comes together. Paige Turco, Isaiah Washington, Henry Ian Cusick, Bob Morley, Marie Avgeropoulos, Lindsey Morgan, Devon Bostick, Christopher Larkin and Richard Harmon also star. Ed Fraiman directed the episode written by Aaron Ginsburg & Wade McIntyre (#315). Original airdate 5/12/2016