Answer: Otto Frank returns to a secret hiding place in which he and his family have lived during the first part of the Holocaust in Amsterdam. His friend, Miep Gies, has saved a diary that his daughter Anne kept during that time. Otto begins reading the diary aloud, and the play follows Anne's thoughts and actions, as well as those of the others hiding in the Secret Annex.
The Franks and another Jewish family try to co-exist peacefully in a small apartment of rooms, hidden behind a bookcase in Otto Frank's office in Amsterdam. They must hide from the Nazis who are exterminating Jewish citizens during the Holocaust. The family members may not make noise or leave the annex for fear of being discovered. Only Miep Gies and Jan Kraler are allowed to know of their existence; they help the families by bringing them necessary items and food.
I hope this helps :)