Sarah's Key, a gripping, if uneven, thriller that unfolds in two separate time frames, jumps back and forth between 1942 in Nazi-occupied Paris and the contemporary French capital.

Like the novel by Tatiana de Rosnay on which it is based, the film by Gilles Paquet-Brenner considers the gravitational pull of past upon present. Excellent performances make the movie effective. Yet the flashbacks have a depth and resonance largely absent from the modern scenes.

At heart, Paquet-Brenner's film is a missing-persons story that reckons how those lost in the Holocaust haunt the living. At its best, it considers the branches missing from so many family trees.

Sarah opens with a scene of tenderness interrupted. A young sister and brother are tickling each other in bed one July morning in 1942 as officers pound the door of the family apartment in the Marais. Is it the Nazis arresting Jews to deport them to concentration camps?

Then Sarah Starzynski (Mélusine Mayance), age 10, hears the men speak in French. Dieu merci, it's not the Germans. When she understands that the French police are executing Nazi directives, she improvises a game to spare her baby brother: Hide in the closet, she whispers, locking him in, secreting the key on her person. I'll be back for you, she promises.

The Starzynskis are dragged to the Vel'd'Hiv, the bike-race stadium where 13,000 French Jews are crammed without bathroom facilities for days before being loaded like livestock on transports to Auschwitz. Paquet-Brenner, a skillful and empathic filmmaker, puts us in the Starzynskis' shoes and evokes the stench of feces, sweat, fear, and anxiety about the boy in the closet.

Cut to Julia (Kristin Scott Thomas), an American-born French journalist in the present day, who initiates a 65th anniversary account of the events at the Vel'd'Hiv. She knows that of the 4,000 children there, none came back from Auschwitz. What this polished professional initially does not know is that in August 1942, her in-laws took over the Starzynski flat, now being rehabbed by her architect husband for himself, Julia, and their daughter.

Mayance is a compelling young actress who grabs your heart with the light-fingered touch of a Fanning sister. Thomas, as always, has passion and compassion to burn. For the movie's first two acts, the fast-paced mystery enthralls with its parallel stories of the determined Sarah and Julia.

When the parallel stories converge, the pulse-pounding film gets perilously close to flatlining. One reason the flashback sequences are more dynamic is that the handheld camerawork conveys a darting sense of threat. The contemporary scenes, shot in cooler colors with fixed camera angles, are more subdued and emotionally detached.

The film benefits from terrific turns by Niels Arestrup and Dominique Frot as French villagers who refuse to collaborate with the Gestapo.

Contact movie critic Carrie Rickey at 215-854-5402 or crickey@phillynews.com. Read her blog, "Flickgrrl," at http://www.philly.com/flickgrrl/