‘We all die alone” is a thought voiced by the famous (Hunter S. Thompson and Orson Welles among them), but it seems to state the obvious. We also all have toothaches alone, do we not?

In Koji Fukada’s “Sayonara” a terminally ill young woman (Bryerly Long) inhabits a blighted land with a humanoid robot as her only live-in companion. Meanwhile, the entire population is heading for the exits. Alone she becomes indeed, with nothing obvious about her situation at all.

Based on a short one-act play by Oriza Hirata, and made in collaboration with Osaka University roboticist Hiroshi Ishiguro, “Sayonara” is both a dramatic experiment and a sci-fi speculation. The heroine and her robot companion (an Ishiguro creation named Geminoid F) are heralds of a future that, given the natural intimacy of their relationship, feels as though it has always been with us.

Previous Fukada films such as “Kantai” (“Hospitalite,” 2011) and “Hotori no Sakuko” (“Au revoir l’ ete,” 2014) were exercises in French arthouse naturalism, though Fukada’s insights into the darker currents beneath his story’s light, even comic surface, were strictly his own.

“Sayonara” shares some of that naturalism, though with a slower pace and a slacker story line that occasionally tests your patience. As the story begins, a near-future Japan has been visited by a nuclear catastrophe so severe that the entire population must evacuate abroad. There is a priority system for departures: If your number is high enough, you go; if not, you stay, even as radiation poisons you.

Tanya (Long), who fled to Japan from South Africa with her parents when she was 10 and has since grown up speaking fluent Japanese, knows that, as a refugee, she is among the last on the list. She lives alone in the house her now-dead parents left her, cared for by the android Leona. Looking like a Japanese woman in her mid-30s, Leona gets around in a wheelchair.

Also, though she speaks English, French and Japanese fluently and recites passages of poetry flawlessly, Leona falls into the uncanny valley between robot and human — and not only because her expressions are slightly mechanical. As an immortal machine, she can follow Tanya only so far in her journey to death.

If the film, like the play, focused on Tanya and Leona’s limited relationship, it would either conclude sooner or drift. Thankfully, Fukada gives Tanya a human friend, Sano (Makiko Murata), and boyfriend, Satoshi (Hirofumi Arai), who relate to her as Leona cannot and broaden the story beyond Tanya’s living room couch, where she spends ever more time as her unnamed illness progresses.

On a drive with Tanya, Sano picks up two young hitchhikers (Nijiro Murakami and Yuko Kibiki) who freely admit that they want to marry so they can evacuate the country together. Their honesty and energy are invigorating. Satoshi visits, makes love and otherwise gives Tanya last moments of normality, but escape is on his mind as well.

Tanya, however, is trapped by both her visa status and her sickness. As her isolation deepens she remembers carefree days with her parents — and her father’s story about bamboo flowers that bloom only once every 100 years. She may not, she tells Leona, live to see them.

Tanya fades into whatever lies beyond, but the process of death continues — and Leona waits.

A member of Hirata’s Seinendan theater troupe, Long developed the role of Tanya in the play and completely embodies it in the film. Seeming to dwindle away before our eyes, she acts Tanya’s dying and death to chill and poignant perfection. And Long bonds convincingly with her inanimate co-star, who, for all her close mimicking of human emotions is no match yet for the real thing. But give her a few more upgrades.