Arrow - S3E8 - “The Brave and The Bold” - The Flash and Arrow disagreement.

Barry travels to Starling City with Caitlin and Cisco, and ends up helping defend Lyla against an assassin determined to get his revenge on her despite Arrow and his friend's best efforts. In the past, Oliver is ordered to acquire information from a prison using whatever means necessary.

"Flash vs. Arrow" is the inaugural Arrowverse crossover event, broadcast on The CW, featuring episodes of the Arrowverse television series The Flash and Arrow. The event began on December 2, 2014, with The Flash episode "Flash vs. Arrow" and concluded the next day with the Arrow episode "The Brave and The Bold". The crossover sees Team Flash (Barry Allen / Flash, Caitlin Snow, and Cisco Ramon) helping Team Arrow (Oliver Queen / Arrow, Felicity Smoak, and John Diggle) take on the boomerang-wielding villain Captain Boomerang, while Team Arrow helps Team Flash confront the metahuman Roy Bivolo.

A crossover between the two series was announced in July 2014, after Barry Allen had been introduced in Arrow's second season ahead of the debut of The Flash. By the following September, the scripts for each episode had been written, with filming occurring shortly before the end of the month and into October 2014. 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 positive reviews, with critics calling the crossover fun, and everything fans could have wanted and more. Critics praised the producers for bringing the two series together while keeping the episodes as self-contained adventures, and highlighted the fight scene between the Flash and Arrow as living up to the hype. Ratings for the episodes were exceptional, with both being the highest watched episodes of each show since their respective series premieres. A subsequent crossover occurred the following year titled "Heroes Join Forces"