Hail HYDRA! is the twenty-first episode of The Avengers: Earth's Mightiest Heroes.
An electromagnetic pulse shuts down the Avengers mansion, after which they find Black Widow at their front door. She reveals to actually not be working for HYDRA, but actually spying the evil organization under Nick Fury's orders. She found out the Cosmic Cube that A.I.M. created for HYDRA can alter reality, and now both organizations are warring over New York for the Cube. Only Fury, who is nowhere to be found, knew about Widow's mission, so she went with the Avengers instead of S.H.I.E.L.D.
The Avengers and Black Widow face the ships of HYDRA and A.I.M. until MODOK's ship is shot down, so the forces of both organizations converge at the area. The A.I.M./HYDRA conflict ends when MODOK is pummeled by a bunch of Dreadnaughts, but before Baron Strucker can reach the cube, Hawkeye shoots an arrow from several buildings away, with Ant-Man on the arrowhead, who at reaching Strucker grows to normal size and punches him.
The battle against HYDRA gets more complicated when Maria Hill, now director of S.H.I.E.L.D., tries to arrest the Avengers for being non-registered superhumans, even though she's told to focus on the current HYDRA problem first.
More Avengers reach Strucker, who tries to use the Satan Claw on Captain America, but the hero manages to get free from his grip. Then Strucker makes a last attempt to reach the Cosmic Cube, but Cap catches it at the same time to Strucker, and the clash of wills makes the cube not work, so Strucker thinks it didn't work at all.
S.H.I.E.L.D. captures the combatants, and Hill vows she will get the Avengers to register and work for her. Black Widow gives a goodbye kiss to Hawkeye and leaves, as Hill doesn't know her true allegiance.