S4, E5: Having received an ominous threat from her old boss, Amunet (guest star KATEE SACKHOFF), Caitlin fears that her past time as Killer Frost may be back to haunt her. Felicity Smoak (guest star EMILY BETT RICKARDS) comes to Central City to help the girls celebrate Iris's bachelorette party, while Cisco, Joe and the guys take Barry out for a night on the town.