The Flash 3×21 “Cause and Effect” Season 3 Episode 21 Inside – Barry (Grant Gustin) takes drastic measures to stop Savitar. Meanwhile, H.R. (Tom Cavanagh) continues to push Tracy Brand (guest star Anne Dudek) to design the trap for Savitar and Killer Frost (Danielle Panabaker) returns with an interesting proposal. David McWhirter directed the episode written by Judalina Neira & Lauren Certo (FLA321). Original airdate 5/9/2017.