The short term memory loss, the polaroid, stuff written all over the hero's torso, him going after the guy(s) who killed his wife/girl friend. That's what the movie is about. One can develop a screenplay but without the premise, there's no screenplay. I say Murugadoss plagiarised Memento. If you disagree, let's agree to disagree.
There's a Japanese movie called "Rashomon" by Akira Kurusawa. A crime is committed and there are 4 witnesses . They all describe the incident in four contradictory ways. This came to be called "Rashomon effect" in movie parlance.
Now Kamal Haasan did movie called "Virumaandi" which used the Rashomon effect. Kamal's wife gets murdered. Both Kamal and the villain gets arrested. In the first half of the movie, a reporter interviews the villain. He narrates a story and all incidents show Kamal's character in a bad light. In the second half, the reporter interviews Kamal. He narrates the same set of incidents in a different light which shows him in a good light and how his wife got actually murdered. Now, this is taking a literary concept created by someone else and creating your own story and screenplay. What Murgadoss did with Gajini is plagiarism.