THE FLASH S3-E21 "CAUSE AND EFFECT" DRASTIC TIMES CALL FOR DRASTIC MEASURES Barry takes drastic measures to stop Savitar. Meanwhile, H.R. continues to push Tracy Brand (guest star Anne Dudek) to design the trap for Savitar, and Killer Frost returns with an interesting proposal. David McWhirter directed the episode written by Judalina Neira & Lauren Certo. Original airdate 5/9/2017.