Mark Ruffalo’s ‘I Know This Much Is True’ is the grimmest TV show of the summer

Mark Ruffalo pulls double duty in “I Know This Much Is True” as identical twins Dominick, right, and Thomas, left.(Atsushi Nishijima/HBO)

Juliette Lewis is ready to sink into her feelings in “I Know This Much Is True.”

The HBO miniseries, based on Wally Lamb’s 1998 novel of the same name, stars Mark Ruffalo as identical twin brothers: one, Dominick, is broken from his daughter’s Sudden infant death syndrome and his subsequent divorce; the other, Thomas, has battled paranoid schizophrenia for his entire life.

The six-episode series, which premieres Sunday, is somehow even darker than it sounds, an exercise in sitting idly by while you watch two men struggle through the despairs of life.

But Lewis, who plays a whirlwind of a grad student hired to translate Dominick’s grandfather’s self-musings from Italian to English, called “I Know This Much Is True” cathartic.

“When you’re a teen and you’re listening to something that synthesizes with your emotionality...it becomes a release,” Lewis told the Daily News, comparing the series to The Cure’s “Disintegration.”

For Dominick, who has spent his life taking care of his brother, his mother and, in his mind, everyone else around him, there is no release. Even people who visit his world briefly, like Nedra, cause him pain.

“Nedra’s like a cyclone. She’s like this little hurricane that passes through this story,” Lewis told The News. “She’s the star of her own orbit. It’s funny to play a narcissist who doesn’t know she’s a narcissist. She’s full of edge and anger, pompous, real pretentious.”

Juliette Lewis plays a graduate student hired to translate an old Italian text.(Atsushi Nishijima/HBO)

“I Know This Much Is True,” whose impetus is Thomas cutting off his own hand in the middle of the library, ostensibly as a protest of the Gulf War, floats back and forth between timelines: the twins’ childhood and an abusive stepfather, college when Dominick meets his future wife (Kathryn Hahn) and even to generations past and their grandfather, whose tome is being translated by Nedra.

The show itself is dark, too, shot in gray skies and brown trees and flannel shirts. Set in Connecticut but filmed in Poughkeepsie, director Derek Cianfrance’s (“The Place Beyond the Pines,” “Blue Valentine”) version of “I Know This Much Is True” is as dismal as Lamb’s original text.

Nedra, who represents barely a blip in Dominick’s timeline, causes him immeasurable pain nonetheless, both in introducing him to his grandfather who turned out to be a cruel, vindictive man and in her own way.

“We meet this personality at the worst month of her life: she’s not sleeping, she’s studying to be a professor, she’s living in this high-brow world of academia oppressed by men and s--t on,” Lewis told The News. “There’s all these neuroses and unresolved issues about men that she brings to the floor the second she meets Dominick.”

Nedra gets added to the list, like his mother (Melissa Leo), his ex-wife (Hahn), his girlfriend (Imogen Poots) and his brother’s therapists (Rosie O’Donnell and Archie Panjabi). Most do not mean to hurt him, but for Dominick, it is inevitable. His quest to figure out who he is, where he came from and why he and Thomas are the way they are was never going to have a happy ending.

“You’re just watching the best of the best really give you this really real-life, beautiful cinematic soul-searching exploration of family dynamics and people’s mythology,” Lewis told The News.

“I Know This Much Is True” premieres Sunday at 9 p.m. on HBO.