Twelve-year-old Siya has been missing for nine months. It's a cold case, but Keshav wants to help his mother, Alia, who refuses to give up.

'My daughter Siya was kidnapped. Nine months ago,' Alia said. The police had given up. They called it a cold case. Even the rest of her family had stopped searching. Alia wouldn't stop looking, though. She wanted to know if I could help her.
Hi, I am Keshav Rajpurohit and I am a disappointment to everyone around me. I live with my parents, who keep telling me how I should:
a) get married,
b) focus on my IPS exams,
c) meet more people and
d) close my detective agency.
 
But Alia Arora, neighbour and ex-model, wanted my help. And I couldn't take my eyes off her face ... I mean ... her case.
 
Welcome to 400 Days. A mystery and romance story like no other. An unputdownable tale of suspense, human relationships, love, friendship, the crazy world we live in and, above all, a mother's determination to never give up.