Crucible

"Crucible" is the fourth episode of the second season, and the twenty-seventh episode overall. It aired on October 30, 2013.

Synopsis
Oliver discovers a man who goes by the name "The Mayor" is bringing illegal guns into the Glades, so Oliver sponsors a "Cash for Guns" event to help clean up the city. Unfortunately, The Mayor crashes the event with his gang and seriously injures Sin. The Canary is furious when she learns her friend is in the hospital and sets out for revenge. Meanwhile, Felicity stuns Oliver with a bit of information about the Canary, Donner asks Laurel to dinner, and John reconnects with an old friend.

Plot
Felicity realizes from Oliver's past encounters with the female vigilante that she has been following Laurel. With this information, Oliver manages to ambush her and discovers that she is Sara, Laurel's sister who was believed to have drowned. It soons becomes clear that while Oliver thought she was dead he knew she survived the initial accident. Meanwhile, weapons shipments are being intercepted by a gang leader calling himself "The Mayor", who assaults a gun rally hosted by Oliver and Blood. This allows Felicity to identify the Mayor, and subsequently track him down. Oliver recruits Sara to assist him in capturing the Mayor. Afterward, a corrupt police officer brings "The Mayor" to an unknown location, where Blood injects him with a green substance that ultimately kills him. Meanwhile, Laurel develops a drinking problem, and refuses help from her father and Oliver. In response, Oliver asks Sara to reveal herself to her family, knowing it will cost him Laurel's friendship. In flashbacks, Oliver is questioned about the bodies of the Japanese soldiers, but when he refuses to answer questions he is taken to an interrogation room where he first learns that Sara is alive.

Production
Preparation ran from August 2 until August 13, 2013. Shooting ran from August 14 until August 23, 2013.

Trivia

 * Sara knows that Oliver is Arrow before being unmasked, due to recognising the origins of his green hood, meaning events during the island might have hinted at the knowledge.