Whatever else this fellow Will Smith — no, not THAT Will Smith — has going on in his life, he tells a helluva yarn.

And that “yarn,” about a mysterious neighbor he barely knows leaving a large and expensive stamp collection with him with an “If anything should happen to me” proviso, instantly drew in documentary filmmaker William Saunders & Co. It sent them on a four year odyssey, with Smith, to track down where these stamps came from, their value, and who this Russian accented fellow, Roman No-Last-Name might be and how he came to have them, and stash them with a near-stranger.

“The Penny Black” is an utterly-engrossing might-be-true-crime docu-mystery, a film laid out like a private eye thriller (they even hire an Archer Agency detective in LA, shades of Ross MacDonald’s Lew Archer), a story with big money, competing agendas, shady characters and a classic “unreliable narrator.

I mean we think, as the filmmakers do, that we can trust this Smith fellow. But can we? The fact that he has no visible means of support in a crazily expensive city, that his dad was a document forger/embezzler and that Will uh, goes through some cash, makes us wonder.

Everything about the film — from its shadowy recreations of what could be home movies of Will’s past to the score (dulcimer plunks that sound like we’re watching an espionage thriller) — screams “Trust NO ONE,” no matter how honest they seem.

The world’s first postage stamp, a British “penny black,” is among the collectibles in the big albums that this Roman fellow dropped in Smith’s lap. According to Smith, anyway. But that 1840 marvel isn’t close to being the most valuable stamp in the collection.

“The Penny Black” lets Smith tell the strange story of how he got the stamps, and then follows efforts he (and the filmmakers) undergo to ascertain their value at stamp shows and auctions, their provenance and just where this “Roman” fellow got off to.

Smith’s matter-of-fact disclosure of how he came into possession of them all gives him a “sketchy” vibe, one that he never quite shakes as months and years go by, he moves a couple of times, takes up with and breaks up with a girlfriend and reveals “gifts” he’s received to prop him up.

“I sold a few stamps,” he jokes, reading the film crew’s mind, and ours. “A gift,” he corrects.

As the years go on and they hire that “Archer Agency” PI and track a down folks who reported a big stamp heist years back, Smith and Saunders build the unseen “missing” Roman into a Harry Lime of “The Third Man” sort of figure — larger than life.

And Smith, who rather casually dismissed the shady seeming nature of their original exchange and shrugged off any idea he might have taken possession of something that could get him arrested or killed, finally seems to fret and worry over what he’s done and what they might uncover.

“I’d hate to put myself in mortal danger over some f—–g stamps!”

Has he?

Dive into “Penny Black,” before somebody options this for a feature film noir, and find out.

MPA Rating: unrated, profanity

Credits: Directed by William J. Saunders. A 1091 release.

Running time: 1:38