Here’s a romantic comedy that teaches at least one truly valuable lesson. No, it’s not about which of the two inevitable suitors the heroine should choose. Or anything about relationships, in fact. All “Letters to Juliet” proves is that if you’re going to make a road movie rom-com, you should invite Vanessa Redgrave to tag along on the journey. She may play a supporting character in the movie, but her grace and charm overshadow all else in it.

As with most chick flicks, the premise of this one is pretty ridiculous. A New York career girl named Sophie (Amanda Seyfried), an aspiring writer working as a fact checker at the New Yorker, is engaged to handsome Victor (Gael Garcia Bernal), a chef who’s at point of opening an upscale Manhattan Italian restaurant. They decide to take a pre-wedding trip to Verona, and Victor’s soon caught up in wine auctions and meetings with cheese suppliers, leaving Sophie with solo time on her hands. So she not only visits the house of Juliet, of Shakespearean fame, but learns of the activities of the Secretaries of Juliet, who gather up the notes that female lovers leave in the cracks between its brick façade and write responses to them. Digging deep into the crevices, she finds a letter dating back to 1957 that was previously undiscovered—from an English student leaving her Italian boyfriend—and promptly pens a reply.

No sooner can one say balderdash than the letter-writer shows up in the form of ethereal widow Claire (Redgrave), accompanied by her uptight grandson Charlie (Christopher Egan), who berates Sophie for dredging up his beloved nana’s old romantic feelings and urging her to come look for her beloved. Soon the three of them are on the road, traveling throughout Tuscany in search of the right Lorenzo Bartolini. It takes them a long while to locate the fellow, happily a rich widower (Franco Nero) who’s cherished the memory of Claire as much as she’s kept his. And along the way, inevitably, Charlie shows his sensitive side and Sophie begins to fall for him.

In several ways “Letters to Juliet” departs from the ordinary rom-com formula. The man-to-be-dumped isn’t a creep, but simply ambitious, and as Bernal plays him, he’s a likable fellow. And not only does grandma accompany the foreordained couple on the journey that brings them together—the trip is about her, and because she’s played with such fragile beauty by Redgrave, one cares far more whether Claire will find Lorenzo than whether Sophie and Charlie will wind up together.

That’s in large part because while Redgrave—and Nero, too—are touching, Seyfried and Egan aren’t. They’re attractive, of course, and amiable enough. But despite all the Shakespearean allusions, they never strike romantic sparks, and under Gary Winick’s listless direction, their scenes together are pretty flat. (The scenes of their auto speeding across the Italian landscape have more energy.) There’s some consolation in the beautiful locations, captured nicely in Marco Pontecorvo’s lush cinematography, but not enough.

The picture could have had a great ending had writers Jose Rivera and Tim Sullivan really decided to upend convention and, after Sophie leaves Charlie to return to Victor, they’d inserted a card reading “50 Years Later” and followed it with a sequence in which the elderly Sophie, now widowed, is shown going off to London to find the Brit again in emulation of the Claire-Lorenzo scenario. But instead they cap off their tale with Sophie’s editor (Oliver Platt) so impressed over her account of Claire’s story that he decides not only to publish it but put her on the magazine’s writing staff, before she dumps Victor and wings back to Europe to reunite with Charlie.

Now there’s fantasy for you.