Heroes Join Forces is the name of the second annual Arrowverse crossover event between The Flash and Arrow. It also served as a set-up for DC's Legends of Tomorrow by introducing new characters who would be featured in that series. The crossover aired over two nights from December 1 to December 2, 2015.
|Vandal Savage arrives in Central City and sets his sights on Kendra Saunders. After Vandal attacks Kendra and Cisco, they turn to Barry for help. Realizing how dangerous Vandal is, Barry takes Kendra to Star City and asks Oliver and Team Arrow to hide her until he can figure out how to stop Vandal. However, things quickly go from bad to worse when a man with wings AKA Hawkman shows up and flies off with Kendra.
Oliver and Barry take Kendra and Carter Hall to a remote location to keep them hidden from Vandal while they figure out how to defeat him. Malcolm arranges a meeting between Vandal, Green Arrow and The Flash that doesn't go as planned. Meanwhile, Felicity, Thea, Diggle and Laurel work with Team Flash to come up with a weapon powerful enough to destroy Vandal Savage.
Malcolm orchestrates a meeting between Savage, Barry, and Oliver. Savage demands they turn over Kendra and Carter or he will destroy both Central City and Star City with the Staff of Horus. Oliver has Barry perform a paternity test on William and it confirms that Oliver is William's father. Confronting Samantha, she agrees to let him see William on the condition he tell no one, including William himself, that he is the boy's father. Felicity learns of Oliver's paternity and feels betrayed because Oliver is still willing to keep secrets from her, so she ends their relationship.Barry and Oliver devise a plan to deliver Kendra and Carter as a ruse to get close enough to Savage to destroy the staff! The plan goes horribly wrong; Kendra's powers fail and Savage quickly overcomes the element of surprise and kills both her and Carter before using the staff to destroy Central City. Barry escapes and runs fast enough to go back in time to the point of the original negotiation before everyone else dies. Barry informs Oliver of his time travel and the mistakes that led to defeat. They change their approach to the plan and Barry is able to steal the staff. He and Oliver use it on Savage, turning him into ashes. Kendra and Carter decide to use their powers to help others in another city. Cisco gives Kendra a tracking device. Oliver agrees to Samantha's conditions for him to see William. Even though Barry encourages Oliver to tell Felicity about William (she never learned about him due to Barry's time traveling) he decides to continue to keep the secret from her. Malcolm collects Savage's ashes while whispering, "You owe me one, buddy."
- Stephen Amell as Oliver Queen/Green Arrow
- John Barrowman as Malcolm Merlyn
- Katie Cassidy as Laurel Lance/Black Canary
- Tom Cavanagh as Dr. Harrison "Harry" Wells
- Casper Crump as Vandal Savage
- Grant Gustin as Barry Allen/The Flash
- Falk Hentschel as Carter Hall/Hawkman and Khufu
- Willa Holland as Thea Queen/Speedy
- Anna Hopkins as Samantha Clayton
- Peter Francis James as Dr. Aldus Boardman
- Jesse L. Martin as Detective Joe West
- Neal McDonough as Damien Darhk
- Jack Moore as William Clayton
- Danielle Panabaker as Dr. Caitlin Snow
- Candice Patton as Iris West
- David Ramsey as John Diggle/Spartan
- Ciara Renée as Kendra Saunders/Hawkgirl and Chay-Ara
- Emily Bett Rickards as Felicity Smoak
- Teddy Sears as Jay Garrick
- Michael Tiernan as Ramesses II
- Carlos Valdes as Cisco Ramon
- Shantel VanSanten as Patty Spivot
- Paul Blackthorne is the only series regular who didn't appear in this crossover.
- Thea's reference regarding superheroes hiding out in a farmhouse was part of Avengers: Age of Ultron, where the Avengers laid low in Hawkeye's farmhouse.
- With the appearance of Hunter Zolomon, (then disguised as Jay Garrick) and Damien Darhk, along with DC's Legends of Tomorrow's Vandal Savage debuting, this is the first crossover to showcase all of the main antagonists during that airing season (aside from Supergirl which was only added into the Arrowverse later).