Though it veers into depression porn once or twice, I Know This Much is True, which debuts on HBO this Sunday, May 10, is an absorbing and superbly acted drama starring Mark Ruffalo and Mark Ruffalo.

Based on the novel by Wally Lamb, the mini-series is about Dominick (Ruffalo) and his identical twin brother Thomas (Ruffalo), who suffers from schizophrenia. Dominick loves his brother, but after an extremely violent act in a library–the story begins at this very moment–he struggles to help Thomas as his own past demons surface.

The production rests largely on Ruffalo’s shoulders, for obvious reasons. Ruffalo, in turn, delivers the best performance(s) of his career. He’s always been a consistent actor, though he’s never demonstrated the range he shows here, especially in the form of Thomas. Thomas is the flashier of the two characters, of course, but Ruffalo’s portrayal of Dominick is equally powerful and significantly more complex. And two episodes in (I was given the first two to review) the show is only beginning to sink its teeth in.

But sink its teeth in it does, even if it uses venom to do so. While presumably the show won’t all be doom and gloom, the first two episodes are full of dark, awful, and depressing events that will shake you to your core–if you can stomach what happens. I was actually disappointed to learn the story was fictional, as my main defense for the show being too dark was that it was based on true events, an incorrect assumption.

As depressing as sections of I Know This Much Be True are, there is presumably light at the end of the tunnel. It’s also clear that writer/director Derek Cianfrance has laid the foundation for a very powerful and entrancing six-episode arc. Cianfrance, who directed Blue Valentine and The Place Beyond the Pines, has a keen eye for absorbing visuals and aesthetics even when telling very personal, intimate stories. Allowed a longform format, I’m intrigued to see how he unpacks and peels away his characters, not just Ruffalo’s both those around him.

I Know This Much Be True may not be the easiest story to stomach, but its rich character work, immersive story, and terrific turns by Mark Ruffalo make this HBO effort worth it. That much, I know to be true.