Daniel Radcliffe has his most useful role ever in “Swiss Army Man,” a dark, existential comedy of dizzying originality.

Because Radcliffe has the title role, that of a corpse who washes up on a beach to “save” a shipwrecked, suicidal and psychically lost soul, played by Paul Dano.

This is no ordinary stiff. Oh no. It has physical and spiritual utility to Hank Thompson (Dano).

Hank has been frantically writing soul-emptying  rescue notes on bottles, making note-rafts out of juice boxes with messages ranging from “Help” and “Save me” to “I’m so bored” and “I don’t want to die alone.”

Failing to revive the dead man, he sinks back into despair, only to discover corpses are buoyant. And their deteriorating guts have…propulsive qualities.

Before you can say “Personal watercraft” Hank is Jet-skiing to the mainland on his new pal, whom he’s decided was named “Manny.”

But it’s only when they’re ashore, somewhere on the littered, remote Pacific northwest that “Swiss Army Man” actually gets weird.

Co-writers/directors Dan Kwan and Daniel Scheinert, adorably billing themselves as “Daniels,” turn two of the cinema’s most eccentric seekers loose on a tale of survival, soul-searching, terror and yes, romance.

Hank is no MacGuyver. But he finds uses for Manny’s flatulence, for the open mouth that captures the rain after Hank has gone to sleep (he’s a water fountain), for his stiffened arms (axes), and so on.

And in Manny, he has his confessor. How Hank ended up so lost isn’t explained (we can guess), but he talks and hums his way through self-psychoanalysis. As he opens his heart, Manny develops something like a pulse. He talks, has no memories, so Hank must explain the universe — emotions, the necessities of life, bodily functions.

“This is what FEAR looks like!”

They hum together, sing-songing through psychotically amusing ditties about their situations and their psyches. The theme to “Jurassic Park” seems triumphant enough in moments like this.

And there’s this woman’s photo on Hank’s slow-to-die phone, a beguiling girl-next-door played by Mary Elizabeth Winstead (of course).

Dano is properly pathetic and manic, and his brother in odd movie choices (“Horns”) Radcliffe manages even more pathos in his limited movement and spare selection of lines of “the life I’ve forgotten.”

It doesn’t add up to much, to be honest. But clever, creative touches abound, as Hank discovers the part of Manny’s body that serves as a compass needle, thanks to a discarded copy of the Sports Illustrated “Swimsuit Issue.”

Whatever its qualities and shortcomings, “Swiss Army Man” makes one promise it most certainly keeps. You have never seen anything remotely like it.

MPAA Rating:R for language and sexual material

Cast: Paul Dano, Daniel Radcliffe, Mary Elizabeth Winstead
Credits: Written an directed by Dan Kwan, Daniel Scheinert. An A24 release.

Running time: 1:35