what is the episodes  plot ?
The crossover sees Team Flash helping Team Arrow take on the boomerang-wielding villain Captain Boomerang.
Flash vs. Arrow
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.

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".
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Flash vs. Arrow [SEP] Introduction
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.