"Heroes Join Forces" is the second Arrowverse crossover event that features episodes of the television series The Flash and Arrow on The CW.

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.

Meanwhile, Caitlin Snow and Harry Wells create a serum capable of temporarily increasing Barry's speed so that he can defeat Zoom.

Jay Garrick initially refuses to test the serum, but is later forced to do so in order to save Wells' life when he is shot by Patty Spivot, who mistakes him for Eobard Thawne.

The plan goes horribly wrong; Kendra's powers fail and Savage quickly gains the upper hand, killing both her and Carter before using the staff to destroy Central City.

[13] The next month, Kreisberg revealed that the crossover would help lay the groundwork for the new team-up, spin-off series Legends of Tomorrow.

Kreisberg and Guggenheim also reassured fans that both episodes would not focus solely on setting up the spin-off series, and that the crossover is still Oliver Queen and Barry Allen having their own adventure.

"[15] In The Flash episode, Caitlin and Harry create a drug called Velocity 6 to make Barry faster against Zoom.

[22] The crossover episodes were scheduled to be shown at an exclusive fan screening hosted by AT&T on November 22, 2015, but it was cancelled to allow more time for the visual effects to be completed.

[34] The Arrow episode attracted season four's largest viewership and 18–49 rating, the best since the previous year's crossover with The Flash on December 3, 2014.

Oliver's paternity storyline felt creaky to Wilkins, and he noted the episodes again strained "to set up the latest spin-off".

Despite these faults, he found "the overall effect of this crossover two-parter ... pretty damn impressive," and the "team-up story ... grand enough to be more than the sum of its parts".

[37] Screen Rant's Alice Walker also noted the annual Arrow and The Flash crossover suffered from trying to set up Legends.

The website's critical consensus reads, "Although the plots aren't particularly inspired, the talented, enchanting casts of The Flash and Arrow make this crossover episode an undeniable joy to watch.

He was critical of the episode's "wonky pacing and story progression" and given its "legitimate two-part storyline" was disappointed it was not more cohesive than the previous year's crossover.

Even so, Scheden said, "there was a lot of fun to be had in seeing both teams cross paths and grapple with an increasingly strange world of heroes and villains" and was looking forward to a focused conclusion.

He noted that the episode wove ongoing storylines "into an expansive DC Universe adventure" calling it "the most purely enjoyable outing of the season so far".

He described the show as "big, adventurous fun" but had the same problem as he had with the previous year's crossover in that when it gave screen time to The Flash "it lost sight of its own identity".

[43] Den of Geek's Mike Cecchini gave the episode 3.5 stars out of 5, saying: "I'm really kind of amazed at just how much work "Legends of Today" gets done without just becoming a massive mess."

The website's critical consensus reads, "This satisfying conclusion to the most ambitious crossover event yet delivers some thrilling surprises and teases out tantalizing possibilities for the wider Arrowverse.

[37] Dave Trumbore felt the Arrow episode tied up the introduction of Hawkgirl and Hawkman and their nemesis Vandal Savage, and effectively set up the launch of The CW's mid-season, spin-off Legends of Tomorrow.

He added that the crossover specials "do a fantastic job of pulling back the camera to reveal the wider world that their heroes and villains share".

[47] Jonathon Dornbush was impressed with Carlos Valdes' performance, saying: "While Oliver naturally takes center stage — and Stephen Amell steps up to the task, arguably giving his best performance on the show yet — the secret MVP of the hour is Carlos Valdes' Cisco, who also happens to steal The Flash's half of the crossover.

Kain praised the shows' creative team for "making the special feel like a legitimate event, as opposed to just a half-assed stab at ratings gold".