So what is the story all about ? It seems like you have heard of this one before. A girl has a strange disease, whereby she can forget entirely what she been doing, even for as large a time as a full day. And somebody is murdered, and the girl cannot even remember what happened. In fact, initially she does not even realize that she is missing a whole Sunday from her life. This girl is Sehar (played by Ayesha Takia). She stays with another girl called Ritu, and one day, almost forcefully, Ritu takes Sehar out to a local club - because Sehar tends to forget where she has been, she prefers not to socialize like this.
Her mother wants Sehar to link up with a policeman, ACP Rajveer Randhawa (played by Ajay Devgn), who is a typical cop in the sense that he can be corrupt when he wants to be. His mother and Sehar's mother know each other and want both of them to get married, but Sehar has rejected the idea since she got to know about the corrupt ways of Rajveer. After the club, Sehar wakes up in her flat, but soon realizes that something is wrong - that she is missing something. She meets a couple of strange characters - one is a taxi driver and one is a struggling actor. After some discussion with Rajveer, they come to the conclusion that she given something in the club because of which she totally went out and forgot what happened on Sunday.
Slowly, Rajveer finds out that Sehar is related in some way to a murder of another girl that happened, and that the taxi driver had left Sehar near where the girl was murdered, and surprisingly the corrupt cop did a quick investigation and finally caught the guy who had murdered his own cheating girl-friend and then later killed the one with whom she was having an affair. Because Sehar was nearby, he thought that she had witnessed the murder, and was after her for that reason. Finally Rajveer manages to solve everything.