The movie "Charly" and the book "Flowers for Algernon" have quite a few differences. Charlie's intelligence grows and drops again in both stories, His romance with Ms. Kinnian, and his Doctor were different in the two stories as well.
In the movie Charly, Ms. Kinnian was already engaged. As Charly became more intelligent thanks to the female Dr. Strauss, Ms. Kinnian became more involved with him. But when Charly started to lose his mental capability, he refused to marry her, purely out of lover her and not wanting her to pity him.
In the short story "Flowers for Algernon" the basic story is about Charly getting more mature mentally. He achieved his goal and started to understand incredible things. Knowledge was coming exceptionally easy for him, but then tragically his increased intelligence failed.
Between the two works, there are also some similarities.
The basic plot of the story remains the same through- out both. The characters are the same, with the exception of Dr. Strauss' gender. Not anything other than that though.
The movie version and the short story of Charly's life have differences and yet all at the same time many similarities. In both works the audience comes to care abut charly and his future.