The cell line (called HeLa, from Lacks’s names) became the basis for a vast amount of medical research, but the movie isn’t about the resulting breakthroughs. It’s about a young author, Rebecca Skloot (Ms. Byrne), who goes in search of the woman behind the cell line and encounters a volatile family, assorted mysteries and all sorts of questions about scientific ethics.

Ms. Skloot of course wrote the book upon which the movie is based, but she is also a character in the story, a white woman intruding on a black family that at first is not inclined to share information about the matriarch or the rest of the clan. The core relationship in the film is the one between Rebecca and Deborah Lacks (Ms. Winfrey, also an executive producer here), one of Henrietta’s daughters, who gradually comes to trust Rebecca and helps her gain access to other family members.

This is Ms. Winfrey’s movie in more ways than one: Deborah, a woman who is both hesitant to learn more about her family history and prone to manic episodes, is a whirlwind of a character. You might think it would be impossible at this point for Ms. Winfrey to escape her own fame and play a movie role, but she is, remember, an Oscar-nominated actress. She makes it easy to forget the rest and buy into her performance, so much so that scenes with Deborah and Rebecca are a bit of a mismatch.

Ms. Skloot has said that she was very resistant to making herself a character in the book, and that seems to carry over into the movie. Rebecca does a lot of observing and cajoling, trying to get people to open up to her, but that’s passive stuff. Only in one scene late in the film, a crackling moment when Rebecca loses patience with Deborah’s mood swings, does Ms. Byrne get to stretch out.

Courtney B. Vance and Leslie Uggams are among the recognizable stars in smaller roles, but the movie is in such a hurry to cover ground that they make only fleeting impressions. There’s a focus problem here: too many threads, not enough time.