This is a different kind of horror story. No bloodthirsty zombies, no pretty-boy vampires, no Chianti-swilling cannibals.

“The Normal Heart,” which premieres Sunday night on HBO, has a far more frightening element: the ugly truth.

Larry Kramer’s Tony-Award-winning story, originally brought to the stage in 1985, may feature fictional characters, but they do a masterful job of representing young, idealistic American males in the early ’80s plagued by AIDS, a disease that hardly anyone wanted to recognize as a deadly force.

“We’re losing an entire generation,” says one of the activists, played with understated grace by Jim Parsons.

It’s a hard film to watch, with a tough back story on how it got to the small screen.

Kramer, a playwright who revels in his prickly reputation, didn’t make an adaptation easy. Even the equally stubborn Barbra Streisand, who owned the rights for 10 years, couldn’t get it done.

Enter “Glee”creator Ryan Murphy, who persuaded Kramer in 2010 that he was the right creative partner.

“It was just a passion project of mine, a play that I have loved even when I was in college,” said Murphy, who joked that he had to take out a second mortgage on his house to secure the rights.

One of his first recruits was Oscar nominee Mark Ruffalo as Ned Weeks, a gay novelist who insists on being the loudest person in the room, a strategy that eventually causes his fellow activists to throw him to the curb.

The divisive character is loosely based on Kramer, with whom Ruffalo got to spend some time before the writer became gravely ill.

“I actually tried to go channel him as much as I possibly could, and honor him and his complexity and his journey,” said Ruffalo of the role, played onstage earlier by Brad Davis, Richard Dreyfuss and Tom Hulce. “His commitment to this movement I deem completely heroic.”

Other members of the all-star roster include Alfred Molina (“Spider-Man 2”), Taylor Kitsch (“Battleship”) and Matt Bomer (“Magic Mike”), who lost 40 pounds in the middle of production to exhibit the ravages of the disease, which, according to the World Health Organization, has so far killed 36 million people.

“We took a very small break and he came back half the person he was and twice the man,” said co-star Julia Roberts. “It was astounding.”

Oh, did we forget to mention Queen Julia?

The actress plays a polio-stricken doctor who attempts, mostly in vain, to rally academics, journalists and the gay community to the cause. Roberts, who appears to have become allergic to steady work, turned the role down twice, largely because she didn’t fully understand the character. Then she stumbled across a documentary about polio.

“I was too young to remember what that experience was like for the world, and the film unlocked the door to who this woman is and where her ferocious, relentless pursuit of correctness comes from,” she said.

Roberts hopes the movie will remind viewers that we are all connected by a thread, one that should make it impossible for us to turn our back on others.

“The movie does that in such a profound way because it’s dealing with a moment that’s so desperate and mysterious and we as a humanity failed each other at the time,” she said. “It’s a great reminder to do better and to stay together.”

Parsons, who, like Murphy and Bomer, is openly gay, hopes the film addresses any critical challenge, not just AIDS.

“It feels like something horrible that happened that has happened before in different ways,” said “The Big Bang Theory” star. “Maybe that’s why a story like this is so important to tell, in the hope that maybe that can be course-corrected.”

Murphy, who is married and recently adopted a child, said his goal is to pay tribute to the warriors who are all too easily forgotten.

“I’ve shown the film to a number of young people in their early 20s who are gay and have no knowledge that they stand on the shoulders of a generation before them,” he said. “What these guys did really paved the way for the life that I have today. I feel indebted to them.”