| "Heroes Join Forces" | |
|---|---|
| Arrowverse crossover event | |
 Promotional poster | |
| Part 1: The Flash | |
| Episode title | "Legends of Today" |
| Episode no. | Season 2 Episode 8 |
| Directed by | Ralph Hemecker |
| Story by | |
| Teleplay by | |
| Production code | 3J5658 |
| Original air date | December 1, 2015 |
| Episode chronology | |
| Part 2: Arrow | |
| Episode title | "Legends of Yesterday" |
| Episode no. | Season 4 Episode 8 |
| Directed by | Thor Freudenthal |
| Story by |
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| Teleplay by |
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| Production code | 3J5808 |
| Original air date | December 2, 2015 |
| Episode chronology | |
| Crossover chronology | |
| Preceded by | "Flash vs. Arrow" |
| Followed by | "Invasion!" |
"Heroes Join Forces" is the second Arrowverse crossover event that features episodes of the television series The Flash and Arrow on The CW. The event began on December 1, 2015, with The Flash episode "Legends of Today" and concluded the following day with the Arrow episode "Legends of Yesterday". The episodes feature characters from Legends of Tomorrow, which was being developed as a spin-off. The crossover sees Barry Allen / Flash and Oliver Queen / Arrow team up to take on Vandal Savage, who is looking for Kendra Saunders and Carter Hall, the reincarnations of Hawkgirl and Hawkman.
A crossover between the two series was announced in January 2015 by The CW President Mark Pedowitz, who stated the network's intention to have an Arrowverse crossover every season following the success of the previous year's crossover "Flash vs. Arrow". By September, the scripts for each episode had been written, with filming occurring shortly before the end of the month and into October 2015. The crossover sees every main cast member of each series who appear at least in their own series, with additional actors and characters also reprising their Arrowverse roles.
The episodes received generally positive reviews; however, the Legends setup received mixed responses. Both series experienced season high viewership despite the ratings being down from the previous year's crossover. A subsequent crossover occurred the following year, titled "Invasion!".[1][2]