The just-released “God’s Creatures” gets a lot of things right, especially the anxious atmosphere of toxic masculinity, but what it might be remembered for most in the coming years is giving the great actress Emily Watson her first lead role in more than a decade. Since 2011’s “Oranges and Sunshine,” Ms. Watson has appeared as the lead in a few British TV movies and series and held supporting roles in feature films, but she has not topped the bill in a major motion picture. Her talent flourishes when she’s given the space and depth to flesh out a complex central character, and her new movie confirms this.

Ms. Watson plays Aileen O’Hara, the middle-aged matriarch of an Irish family who works at a seafood-processing factory in a coastal Irish town. With a sullen husband and his infirm father to take care of, Aileen’s life is depicted as harsh, though there are compensations, such as her grandson, her sensible 20-something daughter, and her female co-workers. When her wayward son returns from Australia after several years, Aileen’s happiness blooms, and she does everything in her power to help him earn a living harvesting oysters, including stealing equipment from her workplace.

When one of her closest cohorts at the factory accuses her son of rape, the slippery slope of her moral compass is tested and the movie dives into major dramatic territory. Like a slow but steadily building piece of chamber music, the movie portrays the different stages of Aileen’s reaction to the accusation, with fear, defiance, surety, doubt, despair, and anger playing across Ms. Watson’s face and especially her large, eloquent eyes. The incredible performance recalls her Oscar-nominated work in another sea-related drama, 1996’s “Breaking the Waves.”

With a spare screenplay and at times mumbled, heavily accented dialogue, “God’s Creatures” leans on visual storytelling to convey the tenor of these characters’ lives and, thankfully, the film’s cinematography is another standout element. Director of Photography Chayse Irvin maintains a tense tone via an impressive range of techniques, including dark-edged compositions, frequent use of silhouette, eerily stately camera movement, ominous window framings, and wide shots with distinctly compartmentalized lighting. Combined, they contribute to the mood of fractured integrity and impending tragedy.

As the accused prodigal son Brian, Paul Mescal continues his ascent as Hollywood’s new heartthrob with real acting chops. Aisling Franciosi as the accuser Sarah proves that her lead performance in “The Nightingale” a few years ago was no fluke: She imbues her character with not only indignation and fire but hard-won wisdom. The directing duo of Saela Davis and Anna Rose Holmer even give her the final scene, a lovely coda that ends the movie on a hopeful note. As this scene played out, though, I couldn’t shake the feeling that they were forgetting their main thematic figure (Aileen) and their best asset: Emily Watson.

How mothers coddle and protect their families, specifically their sons, drives “God’s Creatures” forward, and it’s unfortunate we don’t get a fuller picture of Aileen’s mindset at the movie’s close. Viewers can imagine her next steps, of course, even if they’re tentative, but after a climax that approaches mythic proportion, it seems a cop-out by the filmmakers to not depict how she carries on.

This elision brings to mind an incisive bit of dialogue from earlier in the film, when her just-returned son asks where she would travel if she could go anywhere in the world, and Aileen responds: “I’d rather just reel the years back in and sit you and your father down and make you act like adults.” A lifetime spent dealing with difficult men and appeasing them is encapsulated in this moment, and one hopes in the end that after all she’s been through, she finds some sort of peace. Even if guilt never goes away.