Say goodbye to “Legion.” Or start to, anyway.

The “X-Men”-related Marvel saga of mutant David Haller (played by “Downton Abbey” alum Dan Stevens) begins its third and final FX season Monday, June 24. Once thought schizophrenic, David has been locked in a battle — for self-control, among other things — with ancient enemy Amahl Farouk (Navid Negahban). Causing his own brand of damage while he and his allies have pursued Farouk, David now tries to set things right.

Co-stars Rachel Keller, Aubrey Plaza, Bill Irwin, Jeremie Harris, Amber Midthunder, Hamish Linklater and Lauren Tsai also return for the last round, which opens with a script co-written by “Legion” developer and executive producer Noah Hawley (“Fargo”). “I think endings are what give stories meaning,” Hawley reasons. “I always thought about this as a complete story with a beginning, middle and end. And it felt like a three-act story, so this just felt like the natural place to end it.

“I feel like on some level, what the show is following is this sort of cycle of mental illness,” adds Hawley. “We met David, who had been at his lowest point and tried to kill himself — and (then) he’s in the psychiatric hospital and he meets Syd (Keller’s character) and everything gets balanced out. He’s on his meds, he gets out, everything’s going great for a while. He goes off the meds and spirals down, which is sort of where we find him now. And the question is, can he get back to some kind of good place or is he gone for good? Once we tell that story, it feels like we’re just going back to the beginning of the cycle, potentially.”

In portraying David, Stevens agrees that “great stories have endings. They don’t just stop. When Noah first discussed the story with me, I knew where it was going. I didn’t know how (it would end), but I was promised that it would be weird and beautiful … and it certainly has been. I really like the way this sort of twisted rainbow is emerging. There’s definitely a destination that David wants to get to.”

“Legion” also has given Plaza a unique role — originally intended for a man — in Lenore “Lenny” Busker, David’s friend who died but got renewed life in a different body. The actress claims the last episodes reveal “a whole new side of Lenny. I think this season has been, really, a kind of deep dive into the real human side of Lenny. She’s gotten a second chance at real life now. I’m so lucky to play a character that has so many sides and allows me to explore so many different things.”