The last time we saw Dame Helen Mirren, she was showcasing the “brand new jet ski” that host Jimmy Kimmel promised to the Academy Award winner with the shortest acceptance speech. In addition to being a living legend, Mirren has a refreshing sense of humor that doesn’t preclude taking herself too seriously. That said, with a mantel at home groaning under the weight of one Oscar, four BAFTAs, three Golden Globes, four Emmy Awards, and a Tony, Mirren can afford to be choosy about the roles she accepts.

The Leisure Seeker, Mirren’s current film, must have seemed promising. As Ella Spencer, Mirren plays a determined woman who decides to take her husband John, a retired teacher, on one last road trip, from Wellesley, Massachusetts to Key West, Florida in order to visit the home of Earnest Hemingway. Complications abound. Ella is in the late stages of terminal cancer. John suffers from dementia (we assume it’s Alzheimer’s although it’s never specifically named). Their grown children, not consulted about the journey, are frantic. And, their mode of transport is a vintage Winnebago, long ago packed away in a shed on their property and even longer ago christened “The Leisure Seeker.”

As in any respectable road movie, Mirren and wonderful costar Donald Sutherland set off on an internal journey, in which they learn more about themselves and their relationship than any of the sites along the way. The idea of one last adventure is bittersweet and romantic. The acting is beyond marvelous. And, with our aging population and the challenges of adult caregiving, the situation is timely.

Unfortunately, the story takes a wrong turn early on, and despite truly fine performances from its two leads, it never fully satisfies. In fact, it’s the kind of movie in which you see so much potential that its doubly disappointing when it doesn’t live up to it.

The Leisure Seeker is the first English-language film from celebrated Italian director Paolo Virzi. His credits include another road movie, La Pazza Gioia (released in the U.S. as Like Crazy), in which two psychiatric patients, Donatella and Beatrice, escape together for a life-changing adventure through the Tuscan countryside. In this case, Virzi and his co-screenwriters Stephen Amidon, Francesca Archibugi, and Francesco Piccolo (from the novel by Michael Zadoorian) can’t seem to decide exactly what they want the story to be. (This may be due to too many cooks in the kitchen.) Most of the situations the couple find themselves in are improbable at best, boring and unnecessary more often. John is pulled over for weaving across lanes while trying to open a bottle of soda. Ella has to hitch a ride on the back of a motorcycle when John drives off without her. They run into friendly families at campsites and inept thugs while they wait for AAA to come fix a flat. And the scenery, while pretty, never really changes much. It seems like they go from New England to the deep south in an instant, but then drive by palm trees for days.

Most of all, the tone is too difficult to pin down. There are moments that are funny and moments that break your heart. And while these two different emotions could certainly co-exist, the transitions between them never feel quite right.

What does work, and what delivers that glimpse of how much better the film might have been, is the relationship between Ella and John.

Ella has planned this trip out of genuine love for John, yet she is often exasperated by his memory loss. They happen to run into one of his former students at a Chesapeake theme park. The younger woman is so happy to see him and Ella prepares, out of habit, to take over his side of the conversation. She’s astounded — and eventually angry — that he remembers her name, her best friend’s name, where they both went to college, and the fact that they were always laughing in his class. “You don’t even remember your own children’s names!” Ella snarls as they leave.

Conversely, when John’s memory is working and he says something tender to her, Ella lights up. “I miss me,” he tells her, “I miss you too,” she responds. Seemingly moments later though, he challenges her, demanding to know “Where’s my wife?” “I’m right here,” Ella explains. “No, my wife is young and beautiful and blonde.” Like many dementia patients, John’s past is often closer to him than his present.

In an effort to help him hold on to both his past and present, Ella sets up a slideshow whenever they stop for the night, projecting old Kodachrome images on a makeshift screen. These are sweet scenes, and add dignity to the fading lives of the two characters. (However, Virzi takes it a bit far when he has other campsite guests join them, tentatively inching closer to get a better look.)

The various characters they meet along the way — waitresses, taxi drivers, hospitality workers — never really come to life. Other than the aforementioned would-be thieves, people are kind and generous. Although he can’t remember where they are or where they’re headed, John remembers entire passages of Hemingway and is happy to share them. Meanwhile Ella, who is originally from the south, is friendly and talkative. Unfortunately, she’s also washing down copious amounts of drugs (including OxyContin) with whiskey and wearing a fairly homely wig to hide her chemo-ravaged scalp.