Hua Mulan was a (most likely) real person. She discovered her father's drafting for the war. She understood he was too old and sick to fight. She dressed up as his "son" because the army refused to allow women to fight. She fought in the war, only to be discovered as a woman. Afterwards, she goes home and learns about her father's death. Not knowing how to deal with the grieving of the death and the trauma of war, she eventually commits suicide.
Hua Mulan wants to fight for her disabled father...
The Historical Story
But Hua Mulan is unable to fight because she is female...
So Hua Mulan dressed as her father's "son" and went out to war.