Masumi Hayashi was born on July 22, 1961, in Japan. She grew up in a typical Japanese family, but not much is known about her early childhood. As she grew older, she became a mother of four children.

In her adult life, Masumi worked as an insurance saleswoman. This job required her to meet many people and build relationships with customers. Later, her husband started a business in pest control, which gave her access to various chemicals, including arsenic.

On July 25, 1998, a pot of curry served at a summer festival in the Sonobe district of Wakayama, Japan, was poisoned with arsenic. The quantity used was at least 130 grams, enough to harm over 100 people. After consuming the curry, four people died, including two children and two adults. A total of 63 others became seriously ill from acute arsenic poisoning.

Authorities quickly turned their attention to Masumi Hayashi, a 37-year-old woman seen near the curry pot. Masumi had access to arsenic through her husband, who worked as an insect exterminator. After her arrest, both Masumi and her husband faced additional charges related to insurance fraud. Masumi was also linked to three previous attempted poisonings over the past decade, which were believed to be motivated by her desire for life insurance money.

During her trial, Masumi pleaded not guilty. Despite this, the Wakayama District Court sentenced her to death in 2002. The Osaka High Court upheld her sentence in 2005. Masumi's lawyers argued for her innocence, noting the lack of direct evidence against her. On April 21, 2009, the Supreme Court of Japan rejected her final appeal.

In July 2009, she filed a petition for a retrial. This petition was denied in March 2017. Masumi subsequently appealed the decision, but the Osaka High Court also rejected her request. In June 2021, she filed for a third retrial.

Masumi Hayashi's case is significant because there are questions about whether she was falsely accused. There was no clear motive established, nor was there any direct evidence proving her guilt. Her case received notable public attention and led to a rise in similar poison-related crimes.

In June 2021, two tragic events happened involving Masumi’s family. On June 9, her 37-year-old daughter jumped off a bridge at Kansai Airport, leading to her death and that of her four-year-old daughter. Earlier that same day, her 16-year-old daughter was found dead from blunt force trauma.