Heroes Join Forces

Lore & Background

The crossover was announced in January 2015 by CW President Mark Pedowitz, who committed to annual Arrowverse crossovers after the success of the previous year's "Flash vs. Arrow." By September 2015, scripts for both episodes were written, and filming took place from late September to mid-October 2015. The event was conceived as a two-hour movie split over two nights, with each episode containing important narrative developments for its respective series.

In the plot, Vandal Savage attacks Kendra Saunders in Central City, prompting Barry to bring her to Star City for protection. The teams learn that Savage is an immortal who has killed Kendra and her soulmate Carter Hall across multiple lifetimes. After a failed confrontation leads to Savage destroying Central City, Barry travels back in time to correct the mistakes. The revised plan succeeds: Barry steals the Staff of Horus, and he and Oliver use it to turn Savage to ashes. Kendra and Carter decide to use their powers elsewhere, while Oliver learns he has a son, William, and keeps the secret from Felicity.

The crossover featured main cast members from both series, with additional actors reprising Arrowverse roles. The episodes received generally positive reviews, though the setup for Legends of Tomorrow drew mixed responses. Both series achieved season-high viewership, though ratings were down from the previous year's crossover.

Reader's Guide

"Heroes Join Forces" is significant as the second annual Arrowverse crossover, solidifying the tradition of interconnected storytelling across The Flash and Arrow. It directly set up the spin-off series Legends of Tomorrow, introducing characters and plot threads that would continue in that show. The event demonstrated the network's commitment to a shared universe, with producers describing it as a "two-hour movie" that advanced the narratives of both parent series while laying groundwork for a third. The crossover's use of time travel as a plot device—Barry running back in time to avert disaster—echoed a key element of The Flash's mythology and allowed for a reset of the conflict. Although ratings dipped compared to the 2014 crossover, the event still drew season-high viewership for both shows. The mixed reception to the Legends setup highlighted the challenge of balancing standalone episode needs with franchise-building. The following year's crossover, "Invasion!", continued the annual tradition. The event remains a notable milestone in the expansion of the Arrowverse, showcasing how crossovers could serve both as major television events and as launchpads for new series.

Did You Know?

• Filming for the two episodes occurred from September 25 to October 15, 2015.
• The episodes were originally scheduled for an exclusive fan screening on November 22, 2015, but it was cancelled to allow more time for visual effects.
• The crossover helped lay the groundwork for the spin-off series Legends of Tomorrow.