The sins of the father may be inherited by the son, but in Derek Cianfrance’s deeply poignant, often heartwrenchingly dolorous HBO mini-series “I Know This Much Is True,” our legacies need not define destiny, and forgiveness is a choice. But it’s easy to fall into the dark traps of believing the opposite when life is a constant battlefield. So, the road to arrive at these personal revelations is littered with hardship and tragedy—the handed-down traumas of family, the scars of our past, and the self-destructive narratives we tell ourselves when sustained misfortune and sorrow make living feel like a curse.

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Based on the novel by Wally Lamb and starring Mark Ruffalo in a masterful dual role as disparate twins, Cianfrance’s latest, a towering humanist effort, is deeply textured in its emotional intelligence and insights— about a man struggling to take care of his sick brother while digging into the past of a secret and troubled family history. It’s also a sprawling, but intimate emotional epic about family, sins, forgiveness, and survival, worthy of all the grand pieces of blockbuster cinema, only rendered on smaller, more human canvas, but still equal in power and emotional stakes.  ‘IKTMIT’ is also quite heartbreaking, often emotionally gutting and perhaps hard to watch. But the empathy and humanity Cianfrance and Ruffalo express and radiates throughout this six-episode limited series is profoundly soulful and moving.

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“I Know This Much Is True” centers on Dominick Birdsey (Ruffalo), his mentally ill identical twin, Thomas (Ruffalo again), and the Birdsey/Tempesta family legacy. Thomas is a paranoid schizophrenic which has taken an immense emotional toll on the family over the years. “It’s been a long 40 years,” Dominick laments sarcastically (but honestly) to his therapist, Dr. Patel (Archie Panjabi), at one point, describing his brother as an anchor that has lived around his neck, practically drowning him since childbirth.

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Set in the early ‘90s in Connecticut, Dominick is resentful, bitter, self-loathing, self-pitying, and feels like the family has been dealt a bad hand from the beginning. But you’d forgive him for believing in the inheritance of misery. His disastrous life is a stockpile of major and minor tragedies, and he’s consumed with the hum of a low-key, persistent sadness and PTSD, often manifesting itself in impetuous rage. His mother (Melissa Leo) is dying of cancer and still refuses to reveal the identity of their real biological father—a secret kept so long, Dominick believes in the mysteries’ darkest implication. His marriage to Dessa (Kathryn Hahn) ended in divorce and tragedy, the scars of his emotionally abusive stepfather (John Procaccino) still weigh on him, and his mentally disturbed and delusional twin brother—who believes in all kinds of conspiracy theories— as the series opens on, has severed his right hand in what he sees as a sacrifice to God to atone for his sins.

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Sent to a severe and grim hospital for the dangerously mentally ill, the normally tranquil Thomas is thrown like a lamb to the slaughter, and Dominick makes it his mission to rescue his brother, despite all the pain he has brought his life. Through the various healthcare workers that Dominick meets with in hopes of saving his brother, including social worker Lisa Sheffer (a fantastic Rosie O’Donnell), the sane, but still emotionally-injured brother revisits his past, often told in flashback with the twins depicted twice, once as elementary school kids and brothers in their first year of college (portrayed by Philip Ettinger).

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Much of this introspection is triggered from a critical letter that Dominick’s mother gives him before she succumbs to cancer, grandiloquently written in Italian by his pompous, self-important, and mean-spirited grandfather Domenico Onofrio Tempesta (Marcello Fonte from “Dogman”) about the story of his life. Once translated, Dominick reads through the arrogant autobiography begins to stroll down the memory lane of various pains, while learning more about his lineage, all in the hopes of potentially discovering the identity of his biological father.

But ‘IKTMIT’ isn’t much of an unfolding mystery, nor does it intend to be, even though it has that soft element of intrigue threaded throughout. Cianfrance is working on a much bigger emotional scale about sins, identity, the prisms (and prisons) we see ourselves through, the transgressions we’ve made, and whether there are salvation and redemption to be found in not only forgiveness for others but for ourselves. Weaved throughout on a subtler, but still visible level is the theme of America’s sins, the various systems that fail and doom us, mental health or otherwise, the first Gulf War bubbling in the background, notions of class and dreams, delusions and more.

In its reflections on Dominick’s Italian/Sicilian-born grandfather’s emigration to America, “I Know This Much Is True” starts to take on expansive dimensions of Francis Ford Coppola’s “The Godfather Part II” and its similar historical prequel flashbacks informing who the characters are, where they come from and the American dreams they pursued.

And many of these preoccupations, much like the ideas of family, the scars of childhood, guilt, tragic legacy, and notions of ill-fate in “Place Beyond The Pines,” put Cianfrance back on familiar but not repetitive ground. These kinds of familial explorations of lineage and chance are his bread and butter, and given the scope of six, hour-long episodes, the filmmaker delivers an extensive feast on his consistent human concerns that feels worthy of all its various detours and subplots (not too many, but a few that include actors like Juliette Lewis, Imogen Poots and Rob Huebel). In fact, reading very much like the organic version of an extended film, ‘IKTMIT’ makes the case that Cianfrance’s previous films, ‘Pines,’ “The Light Between Oceans,” and even “Blue Valentine,” as great as they are, would have only benefited from an extended series treatment.

While an unswerving gut punch that could potentially depress some viewers, ‘IKTMIT’ is ultimately hopeful and a masterclass in the complexities of broken people and their struggles. Not enough can be said of just how incredible and devastating Ruffalo is in his two contrasting parts, the rancorous yet melancholy Dominick and the introverted and unhinged Thomas. Suffice to say, there’s likely an Emmy waiting for him and probably a Golden Globe—it’s one of the most convincing and humane portraits of pain, dysfunction, and suffering you’ll see on TV this year.

Despite the material based on another author’s work, Ciafrance— who makes some excellent musical choices throughout including a moody Harold Budd score— makes the material extremely personal and intimate. When the final card dedicating the series to his late younger sister (and Ruffalo’s father) finally appears, it feels touchingly apropos and additionally tender.

Old sins have a long shadow, “I Know This Much Is True,” warns, and the scorching iniquities themselves may have burned a curse onto your family name.  Yet Cianfrance’s brilliant, emotionally harrowing series, for all its heartache, says if we can endure the pain and survive it all, perhaps a new perspective and the light of forgiveness can reveal glimpses of glory and deliverance. [A-]

“I Know This Much Is True” airs on HBO on May 10.