Original: In the second book, Harry Potter and the Chamber of Secrets, Rowling pits Harry against Tom Marvolo Riddle, Lord Voldemort's "memory" within a secret diary which has possessed Ron's younger sister Ginny Weasley.
Simplified: In the second book, Harry Potter and the Chamber of Secrets, Rowling pits Harry against Tom Marvolo Riddle, a memory of Lord Voldemort locked up in a secret diary that Ron's younger sister Ginny Weasley finds in a bathroom.