Even when one's desire to make an impact on the world, or to do "great things," culminates in bettering the lives of others, said ambition still fundamentally comes from a place of self-serving narcissism. It is inherently human for a person to want to be more, and mean more, than what the ultimate reality of mortal life on a planet, in a universe, that has existed for 13.8 billion years provides. 60-year-old Hollywood actor Riggan Thomson (Michael Keaton) is no more important than anyone else around him, and yet the need for acceptance and approval eats away at his consciousness as the deep voice of his most famous onscreen alter-ego—winged superhero Birdman—regularly chimes in with a harsh reality check. Twenty years ago, he was flying high as the title character in the blockbuster "Birdman" franchise, but since choosing to walk away from the series after the third installment he has had trouble reclaiming that same level of success. In a last-ditch effort at a comeback, Riggan has written, directed, financed and plans to star in a Broadway adaptation of Raymond Carver's "What We Talk About When We Talk About Love." Once the show opens, though, will anyone care?

A volcanic meditation on the nature of fame, the realities of aging and the drive to remain relevant, "Birdman or (The Unexpected Virtue of Ignorance)" is an astonishing technical achievement as well as a cutting behind-the-scenes exploration of a stage play in process. Writer-director Alejandro González Iñárritu (2010's "Biutiful") and co-writers Nicolás Giacobone, Alexander Dinelaris and Armando Bo have imagined this project—at once thematically sprawling and geographically intimate—as a spectacularly complex magic trick wherein there does not appear to be a single cut until very, very late into the 119-minute running time. Set over a two- or three-week span in and around New York City's St. James Theatre, the story's passage of time expeditiously moves forward even as the camera's frame of vision doesn't so much as blink.

With previews about to begin on the play, Riggan struggles to hold everything together, to be a guiding force and cheerleading inspiration to his cast and crew while plagued in his dressing room with self-doubt. When an unforeseen injury leaves an empty space in a cast that also includes Riggan's younger, possibly pregnant sort-of girlfriend Laura (Andrea Riseborough) and a film actress nervous about making her Broadway debut, Lesley (Naomi Watts), the production gets a jolt of extra star power when Lesley's current boyfriend, egotistical hot-shot Mike Shiner (Edward Norton), agrees to step into the part. Mike and Riggan butt heads almost immediately over their working methods, but with ticket sales doubling since Mike joined the ensemble, Riggan's manager, Jake (Zach Galifianakis), makes it clear that firing him is out of the question. Amidst personal and public turmoil and an unforgiving New York Times critic (Lindsay Duncan) who has the power to make or break a show into which he has poured his own money and reputation, Riggan cannot help but think that his entire future hinges upon the victory or failure of opening night.

Following a particularly humiliating experience on stage with Mike, a distraught, insecure Lesley asks herself, "Why don't I have any self-respect?" Co-star Laura matter-of-factly replies, "You're an actress, honey." For entertainers of any kind, they bare their souls and try to do their best work, then must release their art into the world for a highly, sometimes viciously, opinionated public to judge. As quickly as they might be on top of the world, they can just as radically be torn down. Riggan Thomson has played this game a long time, but it hasn't gotten any easier. Regardless of his age, the deepening lines on his face, and his receding hair, he wants more than ever to have what he once did and be appreciated for more than simply a character he played two decades ago. To slip into obscurity would be worse than dying. "Birdman" follows Riggan and the people surrounding him—his cast; his crew; his daughter-turned-assistant, recovering addict Sam (Emma Stone)—through dressing rooms, corridors, stages, streets and rooftops in a lingering, deceptively effortless dance between actor and cinematographer Emmanuel Lubezki's (2013's "Gravity") eavesdropping lens. From a purely cinematic gaze, the film is extraordinarily conceived and realized, storytelling of rapturously fluid innovation.

The parallels between Michael Keaton's (2014's "Need for Speed") past and his character's are acutely apparent. As Riggan did with his own masked crusader, Keaton walked away from Bruce Wayne/Batman after taking on the iconic comic book persona in 1989's "Batman" and 1992's "Batman Returns," and since then he has worked steadily but never found the same quality of parts. This all finally changes with "Birdman," which is arguably the best film role he has ever received. Keaton is fearless and unflinching, meeting every daunting demand director Alejandro González Iñárritu makes of him. His contentious interactions with Edward Norton (2014's "The Grand Budapest Hotel"), mesmerizing as the difficult, unfiltered Mike Shiner, radiate with the creative freedom of performers who are invigorated by the work they are doing in spite of how exhaustively choreographed it must have been.

Riggan's relationships with Sam and ex-wife Sylvia bring shading to his portrayal of a man who allowed his profession and ego to define him at the risk of his family. Emma Stone (2014's "Magic in the Moonlight") holds her own against the veteran actors around her, matching them beat for beat and nuance for nuance as Sam, a young woman who has never quite forgiven her dad for not being a more consistent presence when she was growing up. As Sylvia, Amy Ryan (2014's "Devil's Knot") conveys her character's lingering compassion and concern for Riggan even as she can never forget the reasons why she had to leave him. Andrea Riseborough (2013's "Disconnect"), as Laura, and Naomi Watts (2014's "St. Vincent"), as Lesley, are exquisite as two very different actresses bonded by their yearning for validation, while Zach Galifianakis (2013's "The Hangover Part III"), as Jake, reminds of what a fine dramatic actor he can be when he isn't being cast as offbeat funnymen. Special notice goes to Lindsay Duncan (2013's "About Time"), unforgettable in her crucial five minutes of screen time as bitter Times critic Tabitha Dickinson. In one of the picture's most brutal scenes, Tabitha tells Riggan in no uncertain terms that she is dead-set on giving his play a bad review before she has even seen it—an enraging, unfair affront against his life's work with which he uses to turn around and sting her.

Acidic yet empathetic, "Birdman or (The Unexpected Virtue of Ignorance)" rattles and provokes its viewers as more than a rather brilliant technical exercise. It is an experience of subtextual substance, of deep emotion, of cutting dark humor, of magical realism that may or may not exist in the mind of an actor ready to do whatever it takes to be celebrated. "You confuse love for admiration," Sylvia (Amy Ryan) tells Riggan at one point. She might be onto something, but whatever that feeling is—love, admiration, or simply an illusion—he won't stop until he gets it. "Birdman" is one of the year's most distinctive filmic pleasures, but it comes with a single miscue: a final ten minutes that feels curiously tacked on. There is a specific image that Iñárritu should have ended on, one that would have been the perfect closing punctuation on a new modern masterwork, but he keeps going to the detriment of the spell he has conjured and the narrative he has worked so hard to craft. This extraneous epilogue notwithstanding, "Birdman or (The Unexpected Virtue of Ignorance)" is destined to be voraciously talked about and studied for years to come. Perhaps it is imperfect, but it is also unmistakably great.