According to the book, the story of the Deadly Hallows will focus on how Harry, Hermoine and Ron will defeat Voldemort. Before they defeat him, the trio need to find and destroy Voldemort's remaining three Horcruxes (fictional magical objects). The trio isolates themselves to find the magical objects.
Several major characters are killed in the story, and the major twist is the revelation that in order to kill Voldemort, Harry must die too. Harry has a curse that he would have to die to destroy Voldemort.
But Dombledore explains that because Voldemort used Harry's blood to regain his full strength, Harry is protected from any harm that Voldemort could commit, meaning that the Horcrux inside of Harry is destroyed but that he can return to his body despite being hit by the Killing Curse.
Harry returns to his body and kill Voldemort. After the death of Voldemort, Harry and Ginny got married, and sending their sons, James and Albus, off to Hogwarts, with Ron and Hermione, who are now also married with children.