This movie is not about a ghost helping a policeman crack a murder case. This movie is about coming to terms with guilt, grief and pain. It's about understanding the process of healing emotional pain.
Inability to accept and process this pain in a healthy manner can take us into the deeper recesses of the brain, and in the process we can sometimes find the ultimate treasure - the solution to our problem.
Suri is so disturbed, that his inability to process his pain makes him able to access his subconscious where all the answers are stored. While he's able to access information that is not emotionally connected to him (Simran's murder), the biggest challenge is to go deeper inside and safely absolve himself from the guilt of his child's death, which eventually happens as he evolves emotionally by bringing justice to simran.
There is no ghost of Simran. It's all inside Suris head. Unknown to him, he's accessed Simran through his subconscious. It's his subconscious talking to him all the time!
Wow, I watched this movie a couple of years back and understood nothing of this...I watched it just now and this is what I have to say! What a beautiful movie!