Ten-year-old Annemarie Johansen is an active, excitable, and happy ten-year-old. Even though her hometown of Copenhagen is under a strained and fearful occupation by the Nazis, and food, electricity, and heat are scarce and rationed, Annemarie and her family cling to the fact that at least they have one another. Annemarie’s best friend is her downstairs neighbor, Ellen Rosen. Ellen’s family is Jewish, and Annemarie and her younger sister Kirsti often celebrate the Sabbath on Friday nights at the Rosens’ apartment. The two families are very close, and draw strength from one another throughout the difficult, taxing occupation.
As things start growing more dangerous, and local Jewish businesses are shuttered by the Nazis, Annemarie’s Mama and Papa receive a visit from the young Resistance fighter Peter Neilsen. Peter was once betrothed to Annemarie’s older sister Lise, who died recently in a mysterious hit-and-run accident. Now, Peter works with a group which attempts to destabilize and weaken the Nazis who patrol the streets. Annemarie doesn’t believe her family, who are “ordinary people,” will ever be called upon to do the kind of work Peter does—but little does she know that things are quickly changing in Copenhagen.
On the day of the Jewish New Year, Mrs. Rosen and Mr. Rosen come to the Johansens with a plea. Nazi officials have obtained a list of all the Jews of Copenhagen, and have begun rounding them up and arresting them so that they can be “relocated.” Ellen’s parents flee, sheltered by the Resistance—but leave Ellen in the care of the Johansens, who plan to pass her off as one of their own daughters, “Lise,” should the need arise. That very evening, some Nazi officers come knocking at the Johansens’ door. Annemarie instructs Ellen to remove the Star of David necklace she always wears, but when she cannot open the clasp, Annemarie is forced to tear the necklace off for her. The Nazis rouse Annemarie and Ellen from their beds and interrogate them along with Mama and Papa, pointing out Ellen’s dark hair—all of the other Johansens are blonde. Papa pulls a baby picture of Lise from a family album—luckily, the real Lise had dark hair as a child.
The next morning, Mama and Papa tell Annemarie and Ellen it isn’t safe for them to go to school—Mama takes the girls along with Kirsti on a “vacation” to visit her brother, Henrik, in the countryside. Annemarie hides Ellen’s necklace away for safekeeping, and promises her distraught friend that she’ll return it one day. The journey to the countryside is beautiful but fraught with fear, as Nazi officers are stationed even in the idyllic seaside town where Uncle Henrik works as a fisherman.
Things at Uncle Henrik’s house are strange, and someone even brings a casket containing the body of Annemarie’s “Great-aunt Birte.” Annemarie is suspicious, as she knows that there is no Great-aunt Birte. When she confronts Henrik about the strange atmosphere, he confesses that the wake is a ruse, but doesn’t tell her much more—he explains that it is “easier to be brave if you do not know everything.” That evening, many mourners come to the house to sit with the casket—as the night grows late, Peter Neilsen arrives with Ellen’s parents, and Annemarie realizes that all of the gathered mourners are also Jews being protected by the Resistance.
A group of Nazi officers shows up to ask why so many have gathered at Henrik’s house, and though they threaten to open the casket and reveal the entire gathering to be a farce, Mama saves the operation by explaining that the corpse inside the casket may still be infected with typhus. The disgusted officers leave, and Peter opens the casket, which is full of blankets and warm garments, and begins preparing the gathered Jews for their long journey. It has become clear to Annemarie that Uncle Henrik is planning on smuggling them across the narrow sea to Sweden on his fishing boat.
Peter Neilsen takes one group down to the docks, and shortly thereafter Mama leads Ellen and Mrs. Rosen through the forest to the boat. Ellen and Annemarie embrace tearfully, promising to reunite someday. Annemarie, anxious for her mother to return, decides to wait up for her, but is overcome by exhaustion and falls asleep. She wakes in the early morning light to find that her mother has still not come home. When she looks out into the yard, she sees her mother collapsed at the edge of the forest, and runs out to meet her.
Mama is all right, but has sustained a broken ankle after tripping on her way back from the docks. As Annemarie helps Mama up to the house, they both notice something lying on the ground—Mama recognizes the small white packet as an important part of the smuggling operation and urges Annemarie to run as fast as she can with it down to the docks. If Henrik does not have the packet when his ship sets sail, Mama says, all may be lost. Annemarie packs the parcel into a basket along with bread and fruit and sets off into the woods, comforting herself by imagining herself as Little Red Riding-Hood. Indeed, she does run into a big bad wolf of sorts just as she’s about to reach the docks—two Nazi officers and their large, snarling dogs impede her path. The Nazi officers taunt Annemarie and tear apart her basket, but when they rip the packet open they find it only contains a small white handkerchief, and they let her pass. Annemarie delivers the packet to Henrik down at the docks—he thanks her and sends her back up to the house, promising to come home soon.
That evening, Henrik returns for dinner. After the meal he takes Annemarie out on a walk and explains the truth to her: he has indeed been smuggling small groups of Jews out of Denmark and across the sea to Sweden, and the handkerchief—dipped in a specially-engineered solution which dulls the Nazi officers’ dogs’ sense of smell and allows whole boatloads of people, hidden beneath the deck of a boat, to remain undetected—is a vital part of each mission. Henrik thanks Annemarie for her bravery, and assures her that one day she and Ellen will be reunited.
Two years later, Annemarie is twelve, and the war has ended. As Annemarie’s family gathers on their balcony to observe the celebrations happening throughout the streets of Copenhagen, Annemarie retreats back into her bedroom and opens a trunk which holds all of Lise’s possessions—including her unworn wedding dress. Tucked into the folds of the skirt is Ellen’s necklace. Annemarie brings it out to Papa and asks him to repair the clasp so that she can wear it and continue keeping it safe until Ellen and her family return home.