Affluent businessman, Sanjay Singhania (Aamir) is suffering from short term memory loss lasting fifteen minutes at a time. He was hit on the head whilst trying to save girlfriend, Kalpana (Asin), now dead at the hands of gangster, Ghajini (Pradeep).

Due to his amnesia, Sanjay uses post-it notes, tattooed clues on his body and a Polaroid camera helping him remember people and events in his life.

A police officer (Riyaz Khan) unravels Sanjay's story when he gets hold of his diary. He hunts him down for murders committed by Sanjay whilst trying to find his girlfriend's killer.