Willem Dafoe leads us into madness as a sort of performance art in “Inside,” a simple, austere thriller with a highly-polished sheen.

A man, trapped by his greed and artistic/class resentment and passion for that one missing “self portrait” by Schiele, must cope with his circumstances and struggle with his fate in an apartment that was designed to be a self-contained fortress, one that easily becomes his “cage.”

Our unnamed protagonist narrates an anecdote from his youth, about a class assignment to name the “three things you would save” if your house caught on fire. He failed to mention his family or his cat, but made sure to save his sketchbook.

“Art,” he intones, “is for keeps.“

He’s a 50ish struggling artist who turned to art theft at some point. But his latest elaborate heist goes wrong when the unseen tech whiz on his team — somebody else helicoptered onto a penthouse roof — underestimates the electronic security of this luxury flat owned by some sort of oligarch from Kazakhstan. The “inside man” is trapped “Inside,” with a radio-crackled “You’re on your own” the only sign-off from outside.

The claxon from the alarm is deafening. The Medieval church-door he entered through is sealed, and backed by an impenetrable steel sheet. The windows won’t break. The skylight is on a ceiling vaulted so high as to be unreachable.

When he finally works the problem and smashes the alarm, his fate appears to be sealed.

But there’s no “armed response” to this home invasion. No cops, no security from downstairs, no call to the owner, wherever he is, that his alarm was tripped. The CCTV cameras the owner’s entertainment center accesses show only indifference from the front desk guard, the maid eating her lunch in a stairwell, the rich swells going to and fro in the busy lobby.

Our thief is trapped in a spacious flat with a lot of art, much of it flattering the owner. The water to the sinks and such is turned off. The smart fridge works, and alerts him that “supplies are low,” and plays “La Macarena” if he leaves the door open too long.

Water will be an issue long before food is.

Even if he had something to cook, the stove apparently doesn’t work. The wiring, which he has ripped up in efforts to silence the alarm and/or open the door, has very selectively shut off to this or that.

The TV reception is staticky and useless, save for the CCTV feed.

Finding the owner’s Pritzer prize explains the construction. But how in the hell is this thief supposed to get out of there when even security or the police don’t show up after he’s triggered and trashed the alarms?

Dafoe doesn’t turn this into an ostentatious tour de force. It takes a long while for the character to even start chatting to himself, “naming” people he sees regularly on the CCTV — “Jasmine,” he calls the housekeeper (Eliza Stuyck ) who never comes into this apartment.

It takes even longer for him to start dreaming about the owner (Gene Bervoets), conversations about art and philosophy at a swank party.

What we’re treated to in the meanwhile is a try-everything-the-minute-he-thinks-of-it exercise in problem solving. He’s chipping, chiseling and pounding at everything as the days pass, working without tools or with tools he has to devise himself in an apartment that doesn’t have so much as a can opener.

Busting into a pantry delivers canned pate and crackers and such. Getting the cans open earns him his first injury.

As he sketches diagrams and dismantles-destroys the apartment to fashion a wrench here, an entire scaffolding there, we’re not meant to ponder too deeply the fact that no one comes into this place for cleaning, security checks, fix the alarm system or to take up residence as the endless days pass by.

Better to focus on the DIY goggles he fashions, the water problems he struggles to solve.

Filming in Rome under late lockdown conditions, first-time feature director Vasilis Katsoupis makes the most of the chilly but not quite Spartan set and his lone special effect — Willem Dafoe.

An actor who is no stranger to playing artists and crooks, Dafoe lets us see creativity in every “Let’s try that” epiphany. He acts out a sort of doggedness that turns towards philosophical introspection. We note what the character is destroying to save himself, the personal spaces he’s violating and the simple measure almost every trapped person should consider, even the kidnapped Getty kid, he can’t think of or simply avoids.

And we can guess that which he doesn’t ruin.

“Inside” is a minimalist thriller without the visceral, vengeful punch of say “4×4,” which has the captor torment his would-be thief with an idea of murdering him in the SUV he’s just stolen but does not control.

Dafoe is always a wonder to watch. But the picture needed more drama, maybe a touch of humor about its “Martian” styled “work the problem” exercise, and more of a sense of self-awareness in our thief who is trapped “inside” a just deserts parable of his own creation.

Rating: R for language, some sexual content and nude images

Cast: Willem Dafoe, Eliza Stuyck and Gene Bervoets

Credits: Directed by Vasilis Katsoupis, scripted by Ben Hopkins and Vasilis Katsoupis. A Focus Features release.

Running time: 1:45