- Natasha – “How long until you trust me?”
- Bruce – “It's not you I don't trust.”
- ―Avengers: Age of Ultron
Nick Fury sent Natasha to Kolkata, India in order to recruit Bruce to search for the Tesseract. Bruce agreed despite not trusting her, or S.H.I.E.L.D. in general. On the S.H.I.E.L.D. helicarrier, after Loki's brainwashed men caused an explosion in the lab that sent Natasha and Bruce flying through a window, Bruce began to transform into the Hulk. Natasha, whose leg was pinned under fallen debris, attempted to keep him calm, but Bruce still transformed. Natasha managed to free herself and fled in terror from the Hulk. He hit her across a room, and likely would have done more damage if Thor hadn't intervened and gained the Hulk's attention. Despite Natasha being visibly shaken and terrified by the Hulk, the two fought alongside each other during the Battle of New York.
Avengers: Age of Ultron
Since the days of The Avengers, Natasha and Bruce had devised a way to calm the Hulk down enough to turn back into Bruce. The method, called a lullaby, involved Natasha saying, "the sun's getting real low," and then gently touching his arm and hand.
Natasha and the Hulk fought alongside each other to retrieve Loki's scepter from a HYDRA research facility in Sokovia. Though she was still visibly scared of him, she successfully used a lullaby to calm the Hulk down after the battle. On the Quinjet back to Avengers Tower, Natasha tried to convince Bruce that he wasn't a monster, and that he was actually saving lives with his strength.
At the after-party to celebrate their acquisition of the scepter, Natasha flirted with him, though Bruce was hesitant to start flirting with her. She protected him during the ensuing fight with Ultron's bots, telling him not to turn into the Hulk. Wanda Maximoff later inflicted them both with nightmare visions. Natasha was forced to relive her traumatic past, while Bruce was turned into a furious, unthinking, unfeeling monster. Natasha in particular was shaken in her belief that she could be a hero, and begged Bruce to run away with her, though he refused.
Later, when Natasha was captured by Ultron, Bruce rescued her while the others evacuated civilians from the city. Bruce told her that he was willing to run away with her. Natasha, who had returned to wanting to be a hero, kissed him, and then pushed him off a ledge to force him to turn into the Hulk. The two fought in the Battle of Sokovia. Afterwards, Natasha attempted a lullaby, but gunfire from Ultron prevented her from completing it. The Hulk placed Natasha safely on the S.H.I.E.L.D. helicarrier and then stole a Quinjet, flying off to parts unknown. He ignored Natasha's message asking her to come back, greatly saddening her.
When the Hulk saw Natasha in her message on the Quinjet, he was shaken enough to transform back into Bruce.
Avengers: Infinity War
Natasha and Bruce met for the first time in several years at the New Avengers Facility and greeted each other awkwardly. They later fought together in the battle against Thanos' forces in Wakanda. They both survived Thanos' snap to kill half the universe.
- “Fact is he's not like anybody I've ever known. All my friends are fighters — and here comes this guy. Spends his life avoiding the fight because he knows he'll win.”
- —Natasha to Bruce, Avengers: Age of Ultron
- “Hey, big guy. Sun's getting real low.”
- —Natasha's lullaby, Avengers: Age of Ultron
- “She's not the most open person in the world. But with you she seems very relaxed.”
- —Steve to Bruce, Avengers: Age of Ultron