‘I Know This Much is True’ casts Mark Ruffalo as troubled twins with a dark family history

Mark Ruffalo bites into the juiciest of actor buffets in “I Know This Much is True,” portraying twin brothers, one of whom is a paranoid schizophrenic. Once you get past the soapy title, this six-episode limited series offers a grim but gripping adaptation of Wally Lamb’s book, rife with tormented family history and the struggle to overcome the past.

We meet the more troubled brother, Thomas, in grisly fashion, engaging in an act that will force Dominick, a house painter, to take a more active role in overseeing his care. “Thank you for being a good brother to me,” Thomas says pitifully.

For Dominick, the burden of dealing with Thomas has been left to him by their late mother (played by Melissa Leo), and he feels the weight of the obligation. In that respect, the theme bears a considerable resemblance to “Promise,” one of the great Hallmark Hall of Fame movies, with James Garner charged with handling his increasingly disturbed brother, played by James Woods.

Lamb’s 1998 novel — an Oprah’s book club selection — and this adaptation by writer/director Derek Cianfrance (“The Place Beyond the Pines”) contain several additional layers. For starters, there’s the whole twin aspect, and questions about what made one brother turn out this way and the other not. In that, Dominick is guided by a psychiatrist (“The Good Wife’s” Archie Panjabi) despite his resistance, initially telling her, “There’s no use delving into the secrets of the past.”

The story also reaches beyond this generation’s DNA to the family’s dark history, with Dominick learning more about that than he ever wanted to. Finally, there’s the devastating tragedy he experienced in his own life, one that upended his relationship with his wife (Kathryn Hahn) and has undermined future romances going forward.

It is, admittedly, a lot to juggle, and the narrative can get a little unwieldy in spots. But there are plenty of deeply resonant moments, perhaps none more so than an episode that flashes back to the two brothers in college together, as Thomas’ difficulties begin to seriously manifest themselves, playing out against the Vietnam War, and the fear that flunking out of school will lead to him being drafted. (Wars actually frame the story, with the first Persian Gulf war as a backdrop to most of the narrative.)

Despite a titles that evokes that Spandau Ballet ballad, “I Know This Much is True” is raw and intense. The casting is terrific — other supporting players include Rosie O’Donnell and Juliette Lewis — but it’s Ruffalo’s showcase, and he overcomes the clichés about dual roles, creating two distinct characters, while playing Thomas’ quirks in a believable manner. This time, it’s a performance that will make others green, just with envy.

“You don’t just give up on the people you love,” Dominick says, explaining his efforts to help Thomas, as fruitless as they appear to be.

Dominick is an imperfect guide, but “I Know This Much is True” derives its strength from his painful road to forgiveness, even if that means facing hard truths.

“I Know This Much is True” premieres May 10 at 9 p.m. on HBO. Like CNN, HBO is a unit of WarnerMedia.