Medical student, Sunita, is driven by curiosity to study the case of Sanjay Singhania, who is afflicted with short-term memory loss. She runs into him, befriends him, and finds out that he is out to kill a seemingly benevolent citizen, Ghajini Dharmatma. After warning the latter of the impending danger, she subsequently comes across a number of diaries written by Sanjay and attempts to put together a jigsaw puzzle as to how a successful and wealthy businessman became a crazed recluse, who re-lives his past through tattoos on his body, notes and Polaroid photographs on the wall of his Hiranandani Complex flat, and his sole obsession of carrying out his deadly mission - little knowing that Ghajini and his goons are out to erase every bit of evidence he has gathered and thus ensure that he ends up remembering nothing.