In the opening moments of The Zookeeper's Wife, we see a woman in bed, lounging next to a couple of lions.

They're just cubs, but still, as metaphors go, this one is apt.

The woman is Antonina Zabinski (Jessica Chastain), and she is comfortable around dangerous and unpredictable predators, a skill that serves her well as co-operator of the Warsaw Zoo, and that becomes invaluable when the country is overrun and occupied by German troops.

The Zookeeper's Wife tells her remarkable true story: During a murderous occupation that claimed the lives of more than 300,000, she and husband, Jan (Johan Heidenbergh), rescued hundreds of Jews from the nearby Warsaw Ghetto, under the noses of the Nazis.

And of one Nazi in particular — Lutz Heck (Daniel Bruhl), a zoologist assigned by Hitler to crossbreed the zoo's American bison with other animals in an attempt to re-create an extinct German species called the Auroch.

As her husband, Jan, puts it, Antonina "has a way" with Heck and is able to use her influence to disguise and advance Jan's daring, secret project — smuggling Jews out of the ghetto, into the Zabinski home on zoo grounds, and from there to safety.

Sixty years ago, this scenario might have been presented as a post-war genre thriller, building suspense around Antonina's dangerous and fraught position between these two men. Modern directors, though, are aware of the massive moral weight of Holocaust stories, and of their duty to see that these stories are not converted into entertainment.

Director Niki Caro knows it would be tactless and irresponsible to inflate, for the sake of thrills, the suspense surrounding the Zabinskis' operation — the missions were successful, lives were saved, and the director does not sensationalize. And she dutifully records the horrors of the occupation.

But all of this leaves Caro in a tough position — the movie is so dutiful and tactful that it sometimes feels mechanical, constrained by its story rather than inspired by it.

Certainly, much is placed on the shoulders of Chastain, who has become the movies' leading icon of strength and competence, often playing the unerring manager of complex situations — commanding a space station (The Martian) and finding and killing Osama bin Laden (Zero Dark Thirty).

Here again, the stakes are high. Everything is on the line, her marriage, her life, and the lives of the people she is hiding. But Chastain is less steely in the face of things -- no less effective, but more vulnerable and empathetic.

Antonina had prescient insight into the plight of Holocaust survivors. She wanted those in her charge not just to survive, but to do so with humanity intact. At night she brings "guests" to her living room for music and food, and uses the animals she has left to soothe traumatized victims. If Antonina registers strongly, many of those in her care do not. The Zookeeper's Wife also has trouble tracking and marking the passage of time — as a consequence, reunions among separated families lack expected impact.

As does the movie, despite Chastain's good work, and Caro's careful footsteps.