The Black Widow was one of Nick Fury's most trusted agents. Fury's growing concern about HYDRA prompted him to send the Widow on a deep undercover mission as one of their top lieutenants. This required her to betray her partner, Hawkeye, whom she enjoyed working with.
After being exposed to S.H.I.E.L.D., the Widow started feeling bad for betraying Hawkeye. She saved him on HYDRA Island when Baron Strucker attacked him with his Satan Claw, rendered Strucker unconscious with a blast from her wrist blaster, and made up a lie to him after getting the leader of HYDRA to safety.
Later it was revealed that she was in fact working secretly for Nick Fury to uncover the details of the Cosmic Cube that A.I.M. had been developing. She contacted the Avengers for help to keep the cosmic Cube out of the hands of Strucker. Since only Fury and she knows of her undercover status, she is still believed to be a traitor to S.H.I.E.L.D. After the Avengers' intervention in the war between HYDRA and A.I.M. over the Cosmic Cube, Hawkeye offered her to join the Avengers for protection from S.H.I.E.L.D., which she refused, fearing that they too will be at risk and danger for harboring a triple agent from S.H.I.E.L.D. unless her name is cleared by Nick Fury then she kissed him and disappeared in the alley. She later helped Nick Fury, Quake, and Mockingbird forewarn Iron Man of the Skrull Invasion. She was later knocked out by Queen Veranke, but later recovered.
When Galactus came to devour the planet, Black Widow united with Thor, Vision, Luke Cage, and Falcon. She planted charges on the machine while Thor, Vision, Falcon, and Luke Cage fought Air-Walker. After the Herald's defeat, Black Widow destroyed the machine.
Galactus was sent into the Negative Zone and the Avengers were hailed as Earth's Mightiest Heroes.