Set in an anonymous Northern Ireland suburb, “Here Before” initially delights in the disconcerting, run through with curiously dark undercurrents, before giving way to too-common answers. Laura (Riseborough) and her family endured a terrible tragedy many years before, but they’re not really thinking about their lost daughter Josie when a new clan moves in next door, complete with the precocious Megan (Niamh Dornan). Megan is cute and sweet, and she gravitates straight toward Laura, much to the chagrin of her mother (Eileen O’Higgins) and somewhat gruff stepdad (Martin McCann). So far, so good, and then Megan begins asking questions no kiddo should have knowledge of.

It starts simply enough, with Megan asking Laura “who was here before,” a query Laura takes in the literal sense, before her pint-sized new neighbor starts spouting off even weirder asks and observations. Megan tells Laura she’s actually the one who was here before, she’s been to the school, she knows the playground, she even knows the details of the accident that took Josie. Riseborough, so skilled at telegraphing deep emotions with her big eyes and determined mouth, sells the wild ride. What if, Laura wonders, Megan is somehow Josie? Riseborough is good enough to forgive some big, logical leaps. The girls are basically the same age, so classic reincarnation hardily seems like a workable answer, but Laura’s low-simmering pain sells all manner of wild ideas and twists. There’s no question that Gregg and her cast succeed at selling worn-in grief, and while Riseborough is clearly the marquee attraction here, she’s joined by solid costars. (Particular kudos go to young star Lewis McAskie, who brings real texture to the role of Laura’s son.) As Megan continues to worm her way into the family — what this all looks like to her is a question never answered, even when the the film runs right up against that very idea — “Here Before” tries to pile on plenty of plausible deniability. Megan’s mom tries to explain it away — she’s just a kid, prone to fantasy and sometimes even outright lying — but there is unquestionably something else going on. Megan knows more than she should; worse, she seems to enjoy unspooling her little insights to Laura. That’s an avenue worth pursuing, but Gregg puts most of her focus on Riseborough. And who can blame her? If anyone can sell this story and its darkest layers, it’s Riseborough, and the star gives her all to a film that, oddly, becomes less about her as each minute ticks by. That’s a shame: Riseborough has carved out a thrilling niche in weird, creepy, confounding films like “Here Before” wants to be. (Check out her riveting work in the underseen 2018 gem “Nancy,” which also explored the aftermath of a childhood trauma.)

As Gregg ratchets up limp scares — many coming care of a cunning score by Adam Janota Bzowski — “Here Before” shifts its attention to other characters and other motivations, abandoning most of the film’s more out-there ideas in favor of something decidedly more earthbound. It’s also less compelling, and while Gregg offers a cheeky sense of what it really means to gaslight someone, no one will feel as injured by the film’s final-act choices than its audience.

Grade: C+

“Here Before” premiered at the 2021 SXSW Film Festival. It is currently seeking U.S. distribution. Sign Up: Stay on top of the latest breaking film and TV news! Sign up for our Email Newsletters here.