The second season of the American television series The Flash, which is based on the DC Comics character Barry Allen / Flash, sees Barry recognized as a hero in Central City after saving the city, only to face a new threat from a parallel universe in the form of the speedster Zoom, who seeks to eliminate everyone connected to the Speed Force throughout the multiverse.

Grant Gustin stars as Barry, alongside principal cast members Candice Patton, Danielle Panabaker, Carlos Valdes, Tom Cavanagh, and Jesse L. Martin also returning from the first season, and are joined by Keiynan Lonsdale.

Kendra unlocks her abilities and the team decides to regroup in Central City, where Oliver witnesses Samantha Clayton with her son, who is also his.

Jay initially refuses to test the serum, but changes his mind to save Harry when the latter is shot by an unaware Patty, whom Joe later informs about the truth.

[68] However, it was later reported that series co-creator Andrew Kreisberg would be returning to sole showrunner duties at an unspecified time.

[40] In August 2015, Keiynan Lonsdale was cast as Wally West,[26] the unknown son of Joe, and Iris' brother.

[28][78] Discussing the casting of Lonsdale, Kreisberg stated, "Just like when we met Grant [Gustin] for the first time, we instantly knew Keiynan embodied all the heart and courage of a hero.

The reason was initially reported as being due to scheduling conflicts with Shooter; VanSanten later revealed in an interview that she was set to return but "one of the showrunners at the time took a personal disliking to her".

[94] Neely also composed a theme when Gustin as Barry appeared on the eighteenth episode of Supergirl, "Worlds Finest".

[97] In October 2015, Arrow showrunner Wendy Mericle revealed that the producers of the Arrowverse had begun having someone track all the characters and plots used by each series, in order to make sure everything lines up,[98] though Aaron Helbing noted in April 2016 that "sometimes the schedules don't line up exactly...and that stuff is out of our control", such as when Barry is shown using his abilities on Arrow that month, while not having them the same week on The Flash.

[99] The second season of The Flash began to explore the concept of the multiverse, by introducing Earth-2, which features doppelgängers of the inhabitants in the Arrowverse (or Earth-1).

[105][106][104] Casper Crump, Falk Hentschel and Peter Francis James debut in the crossover, as Vandal Savage, Carter Hall / Hawkman, and Dr. Aldus Boardman, respectively.

[49][51][107] Screen Rant's Alice Walker discussed how the annual Arrow/The Flash crossover suffered from also trying to set up Legends, which was "too much to ask from the already crowded storylines and ended up feeling like an exercise in synchronicity, with producers planting more seeds than they could reap.

The crossover event was no longer a fun way to contrast the two shows; it now had to serve the much larger purpose of setting up an entirely new world.

"[109] While no plot details on the episodes were released at the time, Ross A. Lincoln of Deadline Hollywood noted that "the in-universe reason" for the crossover was due to Barry's ability to travel to various dimensions, thus implying that Supergirl exists on an alternate Earth to the Arrowverse in a multiverse.

[112] The episode title was inspired by the World's Finest Comics series, in which Superman would team up with various other DC superheroes, including the Flash.

Berlanti stated that "in a perfect world", the crossover would have featured both Gustin and Amell's Green Arrow, "but logistically that would have been a nightmare to try and do both shows.

Not only did Barry Allen fit perfectly in Kara's world, but actors Grant Gustin and Melissa Benoist had fantastic chemistry together onscreen.

The website's consensus reads, "With distinctive visuals and a terrific cast, The Flash remains one of the strongest comic book shows on television.

"[152] Metacritic, which uses a weighted average, assigned the season a score of 81 out of 100, based on 4 reviews, indicating "universal acclaim".

"[154] Erik Kain of Forbes noted a "very big piece" missing in the absence of Harrison Wells, but felt that the episode was "an excellent start to the sophomore season of the CW's best super hero show.

"[155] Although Henry Allen's abrupt exit was a common point of criticism amongst reviewers, Trumbore nevertheless felt that it was "a small price to pay for an otherwise cohesive, entertaining, and emotionally satisfying episode.

"[157] Angelica Jade Bastién of Vulture said the episode "marries incredible action sequences, amazing direction by Millicent Shelton, some of the cast's best acting (particularly from Candice Patton and Grant Gustin), lots of heart, and just the right number of nods to the comics.

What has never been in doubt, though, is star Grant Gustin's ability to convince viewers that this all makes sense in an emotional, earnest, and often light-hearted way.

With Zoom, The Flash fell victim to a common drama mistake of a contemporary TV era: it prioritized the plot twist over the well-developed character arc.

"[164] Jeff Jensen of Entertainment Weekly gave the season a "B−" grade, calling it "certifiably slumptacular" and said that the "bold" introduction of the multiverse did not meet his expectations.

He also criticized Zoom, saying, "He began as an alluring mystery but lost zip over time" and once his identity was revealed, "became a weak embodiment of generic villainy".

[168] The show was also nominated for three Leo Awards, winning again for Best Visual Effects in a Dramatic Series for the episode "Gorilla Warfare".