400 Days is the tenth novel and the thirteenth book overall written by the Indian author Chetan Bhagat. It is the third installment of the author's popular Keshav-Saurabh mystery series after The Girl in Room 105 (2018) and One Arranged Murder (2020).
Plot
After the incidents of One Arranged Murder, Keshav and Saurabh are living with Keshav's parents in their new apartment in a gated community called Icon. Keshav has quit his job at Cybersafe and prepares for IPS while Saurabh continues to work and is determined to get fit as he is upset with his physical appearance. One day Keshav is called by his parents to meet a woman known as Alia Arora, whom they hope to be a suitable bride for him. They are astonished to see that she also brought her child, Suhana. She is a neighbour living in the adjacent building and asks for his help to find her missing daughter Siya, who disappeared 9 months ago. Though reluctant at first, Keshav is encouraged by his parents to take up the case as Alia had promised huge payments irrespective of the result. Keshav agrees and begins his investigations.
The story then moves to Alia's point of view where she narrates the incidents of the past nine months and her marriage. Alia is married to Manish, a rich businessman, whose family is into jewellery businesses and is one of the richest in the area. Manish's mother is not fond of Alia, as she married Manish without her approval. The entire family always listens to their priest Shastri Ji, who has been associated with them for decades and is like a Godman to them. With his support, Manish convinces his mother to marry Alia. However, Manish's mother constantly offends and bullies Alia, which results in Alia moving out of the joint family and going to live in the Icon. On the day of the incident, Alia, Manish, the children and Manish's brother Timmy and his family all gather at their parents' house for a get-together. The children go to sleep and during the night Siya goes missing. The family questions each and everyone when Suhana suddenly says that she saw a 'Bad Man' who threatened to kill her sister and abducted Siya in the night and recalls a few identities of him. Despite the police's efforts and media coverage, Siya is not found. After 2 months, Alia receives an Amazon package which contained Siya's bloodstained clothes and some locks of hair which belong to her, making everyone believe that Siya is deceased. The police deem the case as a cold case and the family is okay with it as their reputation and business are no longer affected by this incident. However, Alia does not give up and believes that she can somehow find Siya and asks Keshav for assistance.
Keshav and Saurabh access Siya's phone data and unravel many shocking anecdotes. Meanwhile, Alia goes to her hometown Kochi to get some rest and Keshav travels with her to get some details from Suhana, as she is the only witness of the incident. As Keshav gains the trust of Suhana, Alia develops feelings for him as she is neglected by Manish and feels that she is happy with Keshav. Keshav also reciprocates her feelings and they make love multiple times, growing closer. Alia is on the verge of divorcing Manish and she plans to marry Keshav once the legal issues are over. Keshav manages to get more identities of the abductor from Suhana, who says he wore many friendship bands and had a tattoo on his forearm, and also recalled that he smelled like a kitchen, while his hair glowed in the dark.