Helen Mirren, Donald Sutherland hit dead end in road movie 'The Leisure Seeker.'

There is a certain kind of movie cough that signals a very bad prognosis, and Helen Mirren has one in The Leisure Seeker.

She plays Ella Spencer, devoted wife to retired literature professor John (Donald Sutherland), who has advancing dementia. On the day that their son (Christian McKay) comes to confront them about their unmanageable health problems, they take off — in a 1974 Winnebago, hop-scotching from campground to campground on their way to Key West to visit the home of Ernest Hemingway. John fades in and out of lucidity, lecturing strangers on The Old Man and the Sea, or Hemingway in general. Someone asks: Didn't he kill himself?

That prompts the viewer to wonder what happens in The Leisure Seeker when the old man finally reaches the sea.

Director Paolo Virzi often presents The Leisure Seeker as a kind of road-movie lark, but there are darker undercurrents throughout. John, for instance, tells Ella he doesn't want to end up in a home, and instructs that if it comes to that, she's to leave him alone with a shotgun.

>> Read more: Helen Mirren: The tequila-drinking, Jet Ski-riding Dame

We don't even know how long Ella herself will make it. She has that telltale cough, pops an array of pills, and gets sick to her stomach, sometimes breaks down when the burden of caring for her sometimes helpless husband gets to be too much.

Balancing all of this with light-hearted moments isn't easy, but Virzi tries. John drives off without his wife and a biker comes to the rescue, the couple talk their way out of a reckless driving ticket, Ella fends off bandits with the double-barrel shotgun.

Leisure Seeker leans heavily on the charm of its two veteran leads. Sutherland and Mirren work hard to establish John and Ella as a couple worth pulling for, even as we begin to suspect that what they want is to go out on their own terms.

It's a rough ride. John is incontinent, and soils himself. Ella cleans him, acting more as mother than wife, even as she grows sicker each demanding day. It becomes increasingly difficult for Leisure Seeker to manage its varying tones, and ultimately impossible. There are a couple of badly judged scenes near the end — in one, John takes a gun into an old folks home looking for Ella's high school sweetheart, a dreadfully flat bit of slapstick that also undercuts the valiant efforts of Mirren and Sutherland to sell us on the depth and sweetness of their long relationship.

That's nicely illustrated in the movie's best scenes — Ella plugging in a vintage slide projector, showing old family photos on a bed sheet strung over a clothesline at their campsite. Campers gaze, transfixed, at images of lives well lived.