Rain Man: I'm sure all of you know the story. Charlie (Tom Cruise) finds out after his father's funeral that he has an autistic brother, Raymond (Dustin Hoffman), living in a mental institution. Charlie begins by balancing between astonishment and fury: his father left all the fortune to be managed on behalf of Raymond. So Charlie takes his brother out, looking for a way to recuperate the money. What follows is a journey from Cincinnati to L.A. in an old Buick, far from highways, as Raymond is scared by everything. And it is during this journey across America that Charlie learns to understand the condition of his brother and to love him.
At the end the only question that remains for Charlie is why nobody told him about the existence of his brother. And nobody can give him an answer.
There are facts too difficult to share with someone else; too hard to share because too hard to grasp; and as time goes by it becomes harder and harder to tell them to your son.
You thought at the beginning to protect your son from the burden of a mentally ill brother. Even you didn't understand very well at the beginning what should be done. And year after year some sense of guilt added to all that: guilt that you didn't share the terrible truth with your son, fear for him to not find out and get angry on you.
A terrible story, and no wonder some people who watched the movie imagined some jokes, just to balance the tragedy. Look at this video, it's a spoof, Rain Man in 5 Seconds: