According to The Hollywood Reporter, One Day stars Anne Hathaway and Jim Sturgess as people who meet during their 1988 graduation and proceed to meet one day a year for the next 20 years. In When Harry Met Sally... fashion, the story tracks their lives and loves until they realize they were meant for each other.
The British-born Garai will play the woman Sturgess' character marries then ultimately divorces during those years.
The project is based on a novel by British author David Nicholls, who adapted the screenplay.
Production begins in the summer in London.