I quite enjoy the Arrowverse. There are a few crossover arcs, and these are the episodes which they apply to. Maybe this will prove useful to someone else. Flash vs. Arrow - "Flash vs. Arrow" (_The Flash_ S1E8) - "The Brave and The Bold" (_Arrow_ S3E8) Heroes Join Forces - "Legends of Today" (_The Flash_ S2E8) - "Legends of Yesterday" (_Arrow_ S4E8) Invasion! - "Medusa" (_Supergirl_ S2E8) - "Invasion!" (_The Flash_ S3E8) - "Invasion!" (_Arrow_ S5E8) - "Invasion!" (_DC's Legends of Tomorrow_ S2E7) Crisis on Earth-X - "Crisis on Earth-X, Part 1" (_Supergirl_ S3E8) - "Crisis on Earth-X, Part 2" (_Arrow_ S6E8) - "Crisis on Earth-X, Part 3" (_The Flash_ S4E8) - "Crisis on Earth-X, Part 4" (*DC's Legends of Tomorrow S3E8) Elseworlds - "Elseworlds: Hour One" (_The Flash_ S5E9) - "Elseworlds: Hour Two" (_Arrow_ S7E9) - "Elseworlds: Hour Three" (_Supergirl_ S4E9) Crisis on Infinite Earths - "Crisis on Infinite Earths: Part One" (_Supergirl_ S5E9) - "Crisis on Infinite Earths: Part Two" (_Batwoman_ S1E9) - "Crisis on Infinite Earths: Part Three" (_The Flash_ S6E9) - "Crisis on Infinite Earths: Part Four" (_Arrow_ S8E8) - "Crisis on Infinite Earths: Part Five" (_DC's Legends of Tomorrow_ S5E0) Armageddon - "Armageddon, Part 1" (_The Flash_ S8E1) - "Armageddon, Part 2" (_The Flash_ S8E2) - "Armageddon, Part 3" (_The Flash_ S8E3) - "Armageddon, Part 4" (_The Flash_ S8E4) - "Armageddon, Part 5" (_The Flash_ S8E5)