Conversation so far:
What Lord Voldemort in this film?
Voldemort does not appear in Harry Potter and the Half-Blood Prince, although his presence and actions are felt.
How was his presence felt?
he once again declares war, and begins to rise to power once more.
What happens as he gains power?
He murders Amelia Bones of the Department of Magical Law Enforcement, and begins to target members of the Order of the Phoenix, including Emmeline Vance.
What happens next in the book?
Rowling uses several chapters as exposition to establish Voldemort's backstory.

Question: How does she do this?
Self-contained rewrite: How does Rowling establish Lord Voldemort's backstory in Harry Potter and the Half-blood Prince?