Cast;

Aamir Khan as Sanjay Singhania, a rich business man; the chairman of a mobile phone company, Air Voice. He suffers from amnesia which makes him forget all the events that had happened 15 minutes ago.
Asin Thottumkal as Kalpana, a model by profession who gains publicity by proclaiming herself to be the girlfriend of Sanjay Singhania.
Jiah Khan as Sunita, a medical student, who is trying to study the case of Sanjay Singhania and his amnesiac problem.
Pradeep Rawat as Ghajini Dharmatma, a gang honcho and the mastermind of many illegal and criminal ventures.
Riyaz Khan as Inspector Arjun. a police inspector who is investigating the murders by Sanjay Singhania.

Plot;

The film opens with Sunita (Jiah khan), a medical student, and her friends working on a project about the human brain. She wants to investigate the curious case of Sanjay Singhania (Aamir Khan), a (former) notable city businessman, who is reported to have anterograde amnesia. Her professor denies access to Sanjay’s records as it is currently under criminal investigation. Sunita nonetheless decides to investigate the matter herself.
Sanjay is introduced as he brutally murders a man. He takes a Polaroid picture of the man, and time-stamps it “done”. It is revealed that Sanjay has a strange form of anterograde amnesia where his memory is wiped out every 15 minutes. Sanjay uses a system of photographs, notes, and tattoos on his body to recover his memory after each cycle. It is revealed that Sanjay is ultimately out to avenge Kalpana (Asin Thottumkal), and that he is systematically killing the people who were responsible for her death. His main target is “Ghajini” (Pradeep Rawat), a notable social personality in the city.
Police Inspector Arjun Yadav, on the case of the serial murders, tracks Sanjay down to his flat and attacks and disables him. Yadav finds two diaries where Sanjay has chronicled the events of 2005 and 2006. The film enters flashback to 2005 as Yadav reads the diary. Sanjay Singhania is shown as the owner of the Air Voice mobile telephone company. In the course of his business, Sanjay sends his men to meet Kalpana, a struggling model, about putting up a billboard above her apartment. The owner of Kalpana’s advertising firm misinterprets this as a romantic advance, and in view of a possible lucrative Air Voice ad campaign and other benefits, encourages Kalpana to accept the overture. Kalpana thinks of this as an innocent prank, that may fetch her better modeling work, and decides to play out the charade as Sanjay’s girlfriend.
When Yadav is about to read the 2006 diary, Sanjay attacks him and ties him up. He tracks down Ghajini to a college function where Ghajini is the guest of honor. Sanjay snaps some pictures of Ghajini and decides to kill him. He meets Sunita at the function and after some conversation, decides to befriend him. Later that evening, Sanjay mistakenly attacks and kills one of Ghajini’s goons in the parking lot. The dying goon tries to remind Ghajini about an incident that took place two years back. Ghajini is perplexed and fails to remind himself well enough.
In the meantime, Sunita visits Sanjay’s flat and finds Yadav, beaten and bound. She also finds that Ghajini is Sanjay’s target. Sunita finds the two diaries and frees Yadav. Yadav tells her that Sanjay is a known serial murderer. Sanjay arrives suddenly; he remembers neither of them and chases them out. Yadav is eventually hit by a bus, while Sunita barely escapes. Believing Ghajini is in danger, she informs him that Sanjay is after him. Ghajini arrives at Sanjay’s flat to kill him. He destroys all the photographs, notes and scratch off Sanjay’s tattoos. Satisfied that he has neutralized Sanjay by wiping out any trace that connects back to him, Ghajini leaves.
Meanwhile, back in her dorm, Sunita reads the diaries. The film flashes back to 2006. It is revealed that Kalpana had accepted Sanjay’s proposal. This diary ends abruptly. Sunita investigates further, and discovers that Kalpana was traveling for a modeling assignment by train when she came upon 25 innocent young girls being transported to Goa to work as prostitutes and organ donors. She saves the girls who name Ghajini as the ringleader of the racket. Ghajini goes in search of Kalpana. His goons broke into her apartment and wait for her to return. Sanjay arrives there to meet Kalpana. Her last word to him was “Ghajini”. The goons attack Kalpana. Sanjay intervenes but Ghajini hit him over the head with an iron rod. Sanjay’s last vision was to witness Ghajini brutally murder Kalpana with the iron rod.
Sunita, now aware of the shocking truth, finds Sanjay and tells him the truth. He flies into rage and goes after Ghajini. Arriving at Ghajini’s lair in downtown Mumbai, Sanjay brutally kills all of Ghajini’s henchmen and launches into a vicious chase-fight after Ghajini. He is about to kill Ghajini when his memory loss cycle kicks in. Ghajini turns about and stabs him. He begins to taunt Sanjay with the grisly tale of how he murdered Kalpana, when Sanjay, with a sudden burst of last minute strength, overpowers Ghajini. Sanjay kills Ghajini in the same way as Ghajini killed Kalpana.
The film ends with Sanjay, still experiencing amnesia, volunteering at an orphanage. Sunita gives him a small gift which is the cement slab of plaster on which Kalpana and Sanjay pressed their foot when Kalpana moved into a new apartment to remind him of his fond association with Kalpana.