A classified S.H.I.E.L.D. file from 1934 resurfaces, revealing a Hydra-created operative known only as the Siren—Silia Chavez. Trained from childhood into a near-mythic assassin capable of erasing entire teams without detection, she vanished from Hydra records decades ago without a trace. Now, a wave of precision attacks matching Hydra’s earliest methods forces the Avengers to confront an impossible truth: the weapon Hydra lost has returned, and she knows their systems, their tactics, and their weaknesses far too well.