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The Flash S03E21, Cause and EffectS03E21

The Flash · Season 3

The Flash season 3 episode 21: Cause and Effect

Aired 9 May 201742 minRated TV-PG

7.4TMDB · 37 votesMark watched

Where to watch The Flash Season 3 in Australia

The Flash season 3 is streaming in Australia on Prime Video (subscription), HBO Max (subscription) and Stan (subscription), and free on 9Now. You can also buy it on Apple TV from $29.99, Fetch TV from $29.99 and Prime Video Store from $27.99.

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Free

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Availability checked 16 November 2026.

What happens in “Cause and Effect”

Barry takes drastic measures to stop Savitar. Meanwhile, H.R. continues to push Tracy Brand to design the trap for Savitar and Killer Frost returns with an interesting proposal.

Episode summary: TMDB.

Before you watch

  • It is episode 21 of 23 of season 3, so watch episodes 1 to 20 first.
  • It runs about 42 minutes.
  • Directed by David McWhirter, written by Lauren Certo and Judalina Neira.
  • TMDB users rate it 7.4 out of 10 (37 votes).

The Flash trailer

The official trailer for the series (no preview of this episode has been published).

The Flash trailerPlay video: The Flash trailer

Cast and crew of this episode

Director
David McWhirter
Writers
Lauren CertoJudalina Neira
Guest stars
Danielle Nicolet as Cecile HortonRichard Zeman as Lucius Coolidge / Heat MongerLiam Hughes as Young BarrySasha Rojen as Young IrisTobin Bell as Savitar (voice) (uncredited)Anne Dudek as Tracy BrandAndre Tricoteux as SavitarKen Camroux-Taylor as Judge Hankerson

Credits: TMDB.

More from The Flash Season 3

Previous · S03E20I Know Who You Are2 May 2017Next · S03E22Infantino Street16 May 2017
  1. E01
  2. E02
  3. E03
  4. E04
  5. E05
  6. E06
  7. E07
  8. E08
  9. E09
  10. E10
  11. E11
  12. E12
  13. E13
  14. E14
  15. E15
  16. E16
  17. E17
  18. E18
  19. E19
  20. E20
  21. E21
  22. E22
  23. E23

Every episode of Season 3The Flash episode guide

Episode names, air dates, runtimes, stills and ratings: TMDB. Air dates are the first broadcast anywhere; where it streams is our daily Australian check.