It is set in 1947, during the months before and after the independence of India and subsequent division with Pakistan, and is written as diary entries from the perspective of Nisha, a girl who has just celebrated her twelfth birthday along with her twin brother, Amil.

The novel opens on July 14, 1947; it is the twins' twelfth birthday, and to celebrate, their father has gifted Nisha another piece of gold jewelry from her mother's collection and given her brother Amil an illustrated book of tales from the Mahabharata.

Aside from their father, a medical doctor, and the twins, the household includes their paternal grandmother, Dadi, and their cook, Kazi Syed; they live together in a compound in Mirpur Khas.

Hiranandani's father experienced Partition as a nine-year-old boy; although his journey was also from Mirpur Khas to Jodhpur, the specific experiences of Nisha and her family are fictionalized.

Her grandfather was a doctor by profession in the Mirpur Khas city hospital and he left behind up to find a replacement, but a few weeks after, he decided to leave anyway because he was worried about his family.

In 2019, the American Library Association named The Night Diary to its list of Newbery Honor winners, alongside Catherine Gilbert Murdock's The Book of Boy.