Amid the flying magical hammers, psychic blasts and umpteen explosions in Avengers: Age of Ultron lay an epic bromance and an unlikely romance.

The superhero sequel (in theaters Friday) continues the relationship between scientist buddies Tony Stark (Robert Downey Jr.) and Bruce Banner (Mark Ruffalo) from the first Avengers movie. And Banner — and his monstrous green alter ego, the Hulk — finds he has romantic feelings for his co-worker Black Widow (Scarlett Johansson), who has a way of taming the raging beast.

“Pairings are always cool,” Downey says, and the Stark/Banner dynamic found a life off the screen as well with movie fans popularizing the “Science Bros” meme. (Downey and Ruffalo have even had fun with it themselves, sharing goofy pictures of the duo on social media.)

“They’re the two guys who speak science,” director Joss Whedon says of the tech-savvy pair. “It’s a fun little clique to have within the Avengers.”

In Age of Ultron, Avengers Tower is essentially a scientific theme park filled with gadgets and gizmos (“The top 10 floors are like Candy Land,” says executive producer Jeremy Latcham), where the two brainiacs find a home.

Stark “knows it’s always good to check your work with an equal. That’s where him and Bruce really get along,” says Downey, 50. “Also, giving Bruce something to do is a good meditation. To keep things the right shade of green.”

But Stark and Banner birth the villainous artificial intelligence of Ultron (James Spader) after meddling with taboos and questionably ethical discoveries, Latcham says. “They’re working toward something big, they have an idea, and they have the best intentions. But it becomes sentient.”

Ruffalo, 47, says he and Downey work so well together because they’re the “old-timers” in the cast. “There’s an ease there that translates really well and is comfortable for people and fun for us,” the actor says.

“Robert was key in bringing me into the franchise. Certainly when I talked him (before the first Avengers), I was like, ‘I don’t know, Robert,’ and he said, ‘We got this.’ Which is very much the relationship that Banner and Stark have in a strange way.”

Banner continues his quest for normalcy in the sequel, and Ruffalo figures his relationship with Natasha Romanoff is an important part of that. The Black Widow is key to bringing Banner back after Hulk smashes everything in sight, and off the battlefield, they come clean with their feelings for each other.

Natasha has flirted with Stark and Captain America (Chris Evans) in previous movies, but Banner could be her ultimate Mr. Right because both have witnessed and been a part of some pretty heinous stuff, Johansson says.

“At this point in their lives, they finally get this sense of family in and amongst the Avengers, as dysfunctional as it may be, that they both are looking for,” the 30-year-old actress says. “They’re looking at one another going, ‘OK, we put in the hours. Now let’s do something for ourselves because we’re people and we have wants and needs.’

“Yet they’re both tethered to this greater calling that’s even greater than whatever their potential love could be.”

Plus, they work well as a power couple because they’re both “outsiders,” Whedon says. “Neither defines themselves as a hero.”

Their emotional scene in a farmhouse — and those where Stark and Banner are just dudes doing experiments — contain the humanity and intimacy Whedon wants at the heart of an action-packed Avengers movie.

“I start small and get bigger, as opposed to going, ‘Well, let’s all fight and then we’ll figure out why.'”