what episode of arrow does the flash start
. 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".
Flash vs. 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". 
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Flash vs. Arrow [SEP] Introduction
"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".