Kate Winslet and Saoirse Ronan have the quality that all great actors possess, which is that we care about them as soon as they come on the screen. Ammonite intentionally starts off a little slow, but both women appear within the first few minutes, and I immediately knew I wanted to follow them wherever the story may lead. The investment was worth it. This is a striking, emotional film about unhappy people finding happiness together, knowing that it probably can't last. When director Francis Lee gets to the powerful final shot – one that lets the viewer interpret its meaning as they choose – it feels as if we've been participants in the central relationship as much as the characters have.

Loosely based on a true story, Winslet plays Mary Anning, a noted paleontologist in England during the 1840s. She lives with her ailing mother in a small seaside town, selling fossils collected from the beach. Her life changes when Roderick Merchison (James McArdle) comes into her shop, asking to be taught some basic skills. He's not the one who makes an impact, though. That would be his wife Charlotte (Ronan). She's depressed, and when Roderick subsequently goes on a six-week voyage, he asks Mary to check in on her.

Throughout Ammonite, there's talk of Mary chipping away at dirt in order to find the valuable part underneath. Something very similar happens to her. From the beginning, it's clear that she is a profoundly unhappy woman. Later we learn that a former lover, Elizabeth Philpot (Fiona Shaw), broke her heart. To her surprise, Charlotte penetrates the walls she's built up in the wake of that loss. Feelings begin to develop, and when they're reciprocated, Mary gets a new lease on life. Of course, Charlotte is married, so that's a complication.

Ammonite admirably has the patience to let things develop between the women. Rather than rushing the story, the movie asks us to study the looks Winslet and Ronan give each other, to notice their body language. If you watch passively, you might mistakenly believe nothing is going on. Observe carefully, though, and a world of unspoken emotions are happening between the characters. An especially potent scene arrives when Mary and Charlotte attend a recital together and the latter ends up sitting in a different row, right next to Elizabeth. No words pass between them, yet the physical reactions tell us everything we need to know.

Drama intensifies when the emotions finally burst through. Ammonite contains a couple of very explicit sex scenes. What's interesting about them is not the nudity or watching two great actresses doing something graphic. It's that the lovemaking feels as cathartic to us as it does to the characters. The film effectively builds up the silent passion shared by the women, so that when it finally allows them to act on it, the sensation is like a release for us, as well. Few movies are able to achieve that sort of power.

Beautifully photographed and boasting excellent production design, Ammonite has an ambiance that accentuates the story. Period romances can sometimes feel dry and dull. This one is the exact opposite. Winslet and Ronan give it life with their nuanced performances, rendering the movie one of the year's best love stories.

out of four

Ammonite is rated R for graphic sexuality, some graphic nudity and brief language. The running time is 2 hours.