An aging hit man travels to Argentina for a routine job, anxious about missing the birthday of his girlfriend’s 10-year old daughter. The job gets delayed, and he occupies himself in the meantime with tango and a beautiful tango dancer. That’s the premise of “Assassination Tango,” and it sounds like a gimmicky way of jazzing (tangoing) up a classic nice-guy criminal flick. But Robert Duvall — its writer, director and star — is a serious filmmaker who made a good movie a few years ago with “The Apostle” (1997).

Unfortunately, we have to forego the benefit of the doubt we’d like to give to the title character played by Duvall, known stoically here as John J. Anderson. First of all, Duvall is 72 years old. Even if he was a healthy, attractive man with good plastic surgery and a team of stylists and Armani valets to make his “maturity” sexy, 72 is seven years past the retirement age, and it’s only four years younger than the expected age of death.

A wizened hit man with an only-several-decades-younger girlfriend you can take. But when he starts salivating over Luciana Pedraza, the 30-year-old tango beauty (who is, impressively, actually Argentine, but disconcertingly doesn’t seem to open her mouth when she speaks), we enter into the uncomfortable territory of a Humbert Humbert without the eloquence of Nabokov (or even Jeremy Irons).

Duvall, to his slight credit, is aware of this problem. At one point he asks Luciana, “If I were younger, would I have a chance with you?” She, generously twisting our concerns into an attempted reassurance, says something like, “Old, old, you’re always thinking about this age thing!” And then, in what could be a kind of progressive non-age-discriminatory big-heartedness, she says, “You have a chance.” You can almost feel Duvall’s little wheels turning, wondering where he can get Viagra in Buenos Aires.

Aside from the dirty old man issues, the movie is, in a weird way, well-acted and well-crafted on a moment-to-moment basis. But the moments just don’t link together. A good deal of time is spent on the international intrigue and blacksuited criminal aspects of the movie, but any of the tensions developed there end up dissolving into the deflated, happy ending. And you just can’t take Duvall seriously when he’s always wearing his Italian sunglasses, be it to his “daughter’s” elementary school, to an early-bird special at the Olive Garden or to a swank Argentine dinner club.

It’s somewhat endearing that Duvall seems to have tried to combine two of his personal fascinations (hit men and Argentine tango) in a film; unfortunately, we end up with oxymoronic nonsense like Gertrude Stein’s “tender buttons,” and are left squirming before the geriatric tango of Hollywood-meets-art-flick gone wrong.