Oliver discovers a man known only as the Mayor is arms trafficking in the Glades, so he sponsors a 'cash-for-guns' rally to help clean up the city. Unfortunately, the Mayor crashes the event with his gang, seriously injuring Sin - and when the Canary finds out her friend is in hospital, she sets out for revenge. Meanwhile, Felicity has some stunning information for Oliver, Donner asks Laurel to dinner, and Diggle reconnects with an old friend.