Central City police scientist Barry Allen arrives in town after a super-strong thief steals a piece of scientific equipment. Sin asks Roy for help in finding a missing friend, and in the past Oliver and his friends search for a way to cure Slade.

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About

This episode marks the first appearance of Barry Allen, a.k.a. "the Flash." Arrow will tell its version of the character's origin story over three appearances this season: a two-parter (208-209), followed in the spring by a planted pilot for a Flash spin-off show (episode 220).
Actor Grant Gustin (Glee, 90210) has been cast as Barry Allen (story).
From The CW:

BARRY ALLEN COMES TO STARLING CITY - A seemingly impossible robbery at Queen Consolidated's Applied Sciences Division brings Central City police scientist Barry Allen (guest star Grant Gustin) to town. Citing a similar case back home, Barry offers to help Oliver (Stephen Amell) and team with the investigation. Oliver senses there is more to Barry than meets the eye, but he's distracted by the similarities between this current case and something that happened on the island. Meanwhile, Felicity (Emily Bett Rickards) takes a liking to Barry, which doesn't go unnoticed by Oliver. Sin (guest star Bex Taylor-Klaus) asks Roy (Colton Haynes) for help when a friend of hers goes missing. Roy is surprised when Thea (Willa Holland) not only encourages him to help, but joins the search. Unfortunately, Sin's friend is connected to Brother Blood (guest star Kevin Alejandro), and their search ultimately gets one of them seriously injured.