Kogawa's Obasan is set in the early 1970's in Alberta, Canada. The protagonist, Naomi Nakane is a middle school teacher in her mid-thirties, one morning she receives a call telling her that her uncle, Isamu is dead. Naomi goes to visit her Obasan (Japanese for aunt) who is now alone after her husbands death. Obasan is seemingly calm despite the sad news. Naomi persistently tries but fails to engage her in conversation. She asks Obasan to rest but she refuses to and climbs up to the attic instead.
As the two climb up to the attic, Naomi remembers her mother who left her family so abruptly many years ago. Thinking about her mother brings back memories of her childhood during the war. In the attic Obasan comes across a package from Aunt Emily who is Naomi's mothers sister. The package contains dozens of letters, seemingly random papers, journals and books. Going through the package brings back more memories from Naomi's childhood, her old house, the neighbor who molested her, her mother and other such painful remembrances.
All the struggles faced by Naomi's family are documented in the papers and journals in Aunt Emily's package. The troubles faced by the Nakane's are reflective of the plight of Japanese Canadians during the Second World War. Their houses were looted, possessions confiscated and some were even taken to labour camps in remote parts of the country. As these atrocities were going on Naomi's mother had to move back to Japan to take care of her relatives. Many other members of the family were sent to internment camps. With only Obasan, Naomi and her brother Stephen left, they decide to move to Slocan, a ghost town which was relatively safer than Alberta. A while after they move Obasan's husband returns from the internment camps and joins them in Slocan.
Naomi does not remember much about Slocan but Aunt Emily's journals and letters tell her that their time there was marked by struggles and difficulties. Slocan was a deeply racist town and everyday was a struggle to live through. On the morning that the war is declared over, Naomi's father returns to the family in Slocan. Just as the family settles into a comparatively peaceful routine, both her father and Stephen contract tuberculosis. Naomi's father does not survive the disease.
Without warning, one day the government declares that everyone living in Slocan is to evacuate the town immediately. All five members of Naomi's family are forced into a single roomed hut on a farm in Granton. On the farm the entire family is forced to do grueling labour for their food and shelter. Slowly peace returns to the country and Japanese Canadians are freed. Naomi and her family leave the hut and move into the town.
On the day of Uncle Isamu's funeral all of the family members arrive to pay their respects. Stephen and Aunt Emily bring an English minister who reads some of Aunt Emily's letters out loud. One such letter was from Naomi's grandmother in Japan to her grandfather. Both she and Naomi's mother were caught in the Nagasaki bombings. Her grandmother lost Naomi's mother during the bombings but she found her the next day terribly burnt and disfigured with maggots all over her body. Naomi's mother did want her children to know of her death which was why it was kept a secret for all of her life. The entire family is shaken by the new revelation
The book ends with Naomi in Obasan's valley with the flowers and mountains before her. There, she can feel the presence of her uncle and her mother.
