On June 12th, 1942, a young Jewish girl named Anne Frank receives a diary for her 13th birthday. She's thrilled with the present, and begins writing in it straight away, addressing many of her entries to an imaginary friend named Kitty. Anne explains that although her family is from Frankfurt, Germany, she now lives with her mother, father, and older sister (Margot) in Amsterdam, Holland. Her father is the director of the Dutch Opekta Company (a manufacturer of jam-making products). Anne leads a normal life – she plays with her friends, has a number of teenage boy admirers, and worries about her grades – until her family is forced to go into hiding when Margot receives a call-up notice from the SS.
With the help of several of Mr. Frank's employees, Anne and her family take refuge in the Secret Annex, a suite of rooms in a house adjacent to the Opekta warehouses and offices. They are soon joined by the van Daan family: Mr. van Daan, Mrs. van Daan, and Peter van Daan. Peter is 16, and Anne finds him dull and uninteresting. Life in the Annex isn't perfect – there are a lot of quarrels, given the close quarters – but Anne realizes that it's far better than life on the outside, where many of the Frank's Jewish family and friends are being sent to concentration camps. After a few months, a middle-aged dentist named Alfred Dussel joins them in the Annex, where he shares a room with Anne. Anne finds herself at loggerheads both with Mr. Dussel and with Mrs. van Daan – they criticize Anne's chatty ways. Anne feels isolated, and she wonders if she'll ever have a friend she can truly confide in. Anne is haunted and guilt-stricken by dreams of her old friend Hanneli Goslar – to Anne, Hanneli represents the suffering of the Jews.
A year and a half goes by, and Anne becomes a real teenager – she begins menstruating, and she begins pondering questions of sexuality, love, and personal identity. Although she initially found Peter uninteresting, Anne finds herself suddenly drawn to him – could it be that Peter could be someone she can confide in? One night, Anne has a life-changing dream about Peter "Petel" Schiff, a boy she was in love with during her childhood. After she has the dream, Anne finds herself feeling more independent and adult. She begins spending more time with Peter, and she finds that her romantic feelings for Petel blend into her feelings for Peter. Anne and Peter fall in love for a time, though Anne ultimately decides to distance herself from him. Anne decides that she wishes to become a writer and a journalist when she grows up, and she's excited to think that her wartime diaries might one day be published as a book. Anne's final entry in her diary finds her pondering who she really is – will she ever be able to reveal her "second" self to the world? The self that's serious, sensitive, and not at all like her chatty exterior? The diary ends abruptly in August, 1944.
An afterward explains that the denizens of the Secret Annex are betrayed to the SS. Anne and Margot were eventually transported to the Bergen-Belsen concentration camp, where they both die of typhus just a month before the camp is liberated by the Allies. Anne's father is the lone survivor of the Frank family, and his former employee, Miep, gave him Anne's diaries, which she found in the Annex after the SS ransacked it.