Ari Aster’s “Beau is Afraid” is an inscrutable thriller scripted and directed with the confidence of a filmmaker whose “Hereditary” and “Midsommer” upped the intellectual ante on modern horror, but edited with the audacity of a Next Big Thing who’s been reading his own glowing reviews.

It’s a cumbersome, ungainly journey through phobias and mommy issues — sprinting out of the gate in the first act, struggling to come to a conclusion in the fourth — a movie whose “Truman Show” ending has a whiff of “Defending Your Life,” with Patti Lupone in the Faye Dunaway/”Mommy Dearest” role.

That makes it a film everybody is going to be talking about, many will try to dissect and few will want to sit through a second time. It’s stress-inducing and patience-testing, an intimate story told in epic scale and at epic length.

But no, you don’t need to see it in IMAX, no matter what the A24 “event” hype.

The brilliant first act throws us into a paranoid’s vision of The Big City, a Heironymous Bosch hellscape straight out of Fox News depictions of New York, Chicago and D.C., depictions meant for rural folks who would never go there anyway.

Joaquin Phoenix is the title character, a quivering mass of insecurities, on medication and in therapy (Stephen McKinley Henderson is his quick-with-a-“script” shrink). And when we see the world the way Beau does, we get it.

He sprints past murderous, tattooed crazies to get to the store or his psychotherapy appointments, and dashes past a street market where assault rifles are sold and surgeons, still in their bloody scrubs, sip espressos and cops draw on any citizen fearful enough to seek their help.

Soul-sucking anxiety is the only sane response to this Kafkaesque nightmare, and that’s how Beau lives — secluded in his beseiged apartment building, looking for some way, any way, to get out.

Maybe that planned trip to visit his rich CEO mother will do the trick, his therapist hopes. But even the “realistic expectations” that his shrink wants him to embrace include a killer caveat about the woman who gave birth to and raised him.

“Do you ever wish that she was dead?”

To make that visit, Beau must contend with increasingly insane notes slid under his apartment door about the “noise” he’s making (he isn’t), notes that lead to the unseen threatener cranking up CONCERT level music that pounds Beau awake through one more miserable night.

He oversleeps. A loony confluence of events conspire to prevent that trip, but that’s nothing compared to the guilt trip his mother gives him on the phone before hanging up.

That missed visit drives the narrative, as something happens to Mom and Beau’s efforts to be there for her are thwarted by an accident, other violence, the reluctant-to-release-him religious couple who take him in (Amy Ryan and Nathan Lane) and other detours.

Flashbacks are how Beau shows us how he turned out like this, assorted childhood clashes with his single Mom (Zoe Lister-Jones), nightmares about things he might have witnessed back then and a cruise with her where he meets the first and only girl ever to take an interest in him. Elaine’s tween-aged “Wait for me” also circumscribed his life.

Beau’s odyssey to be by his mother’s side has him chased and shot at, tumble into a traveling theater troupe’s forest-idyll production of a play that seems to be about his life, has assorted testy calls with his mother’s lawyer (Richard Kind) and features fresh injuries, physical and psychological, which are the last things this 50ish sack of insecurities needs.

Phoenix is absolutely perfect in the part, and the casting — up and down the line — is deliciously on-the-nose. Ryan is nurturing until the moment calls for her to snap, Lane is at his most unctuous. Parker Posey plays childhood crush Elaine as a libidinous adult. Bill Hader pops up as a delivery man with bad news and Lupone is as imperious and delusionally martyred as you’d expect, playing the older version of Beau’s archetypal Jewish mother

But the forest idyll manages to be inventive (animation, and amateur theatrics) and a tiny bit revealing and stunningly boring at the same time. Aster gives few sequences much urgency, despite Phoenix breaking into panicked sprints here and there. The “recovery” with the still-grieving parents of a fallen soldier (Ryan and Lane) includes an over-the-top rebellious teen (Kylie Rogers) and one of their son’s deranged comrades, named Jeeves (Denis Ménochet).

The violence, an Aster trademark, shocks and repels in between interludes where we think Beau is getting help, getting answers and might even get better.

I’m guessing the average viewer can appreciate much of what’s going on and maybe even take some pleasure in figuring out what Aster is going for here. But “Beau is Afraid” has an indulgent, opaque air that combined with scenes that go on past their payoff makes it an unpleasant, almost assaultive experience in its violent moments, repetitive and dense in others.

Darren Aronofsky’s equally ambitious “Mother!” was an hour shorter, after all.

Performances aside, there just aren’t enough “Mommy Issues” here to justify the tedium of a movie that challenges you and wears you out but doesn’t deliver a payoff satisfying enough to make it worth this much of your time.

Rating: R for strong violent content, sexual content, graphic nudity, drug use and language.

Cast: Joaquin Phoenix, Amy Ryan, Nathan Lane, Stephen McKinley Henderson, Zoe Lister-Jones, Richard Kind, Parker Posey and Patti Lupone.

Credits: Scripted and directed by Ari Aster. An A24 release.

Running time: 2:59