Original: In Harry Potter and the Deathly Hallows, Harry, Ron, and Hermione leave Hogwarts to complete Dumbledore's task: to search for and destroy Voldemort's remaining four Horcruxes, then find and kill the Dark Lord.
Simplified: In Harry Potter and the Deathly Hallows, Harry, Ron and Hermione leave Hogwarts to complete Dumbledore's task: to search for and destroy Voldemort's remaining four Horcruxes, and then find and kill the Dark Lord.