9. What We Leave Behind

Complete story — includes the ending

Prometheus obtains further intel about the team from Evelyn. He then attacks and hospitalizes Curtis, injecting him with a tuberculosis vaccine developed by Justin Claybourne, a corrupt pharmaceutical manufacturer named on Oliver's former kill list. Flashbacks show that Oliver killed Claybourne after discovering that he financed a tuberculosis epidemic, then raised the price on his drug to boost his company's profits. When the team tracks down Prometheus, Evelyn reveals her true allegiance and escapes with Prometheus. Investigating Prometheus on his own, Billy sends information he discovers to Felicity just before the villain captures him. The information turns out to be about Claybourne's illegitimate son, who may now be seeking retribution. Oliver deduces that Prometheus is at the former office building of the corporation that created the epidemic and goes there alone. He finds that Prometheus has staged it to resemble Oliver's prior attack. Oliver kills Prometheus, only to discover he has actually killed a gagged Billy, whom the real Prometheus set up as himself to trick Oliver. Curtis' husband, Paul, leaves him after discovering Curtis is a vigilante; Felicity mourns Billy's death; Diggle is recaptured; Oliver encounters an apparently alive Laurel Lance inside the lair.