Lucius Malfoy planted Tom Riddle’s diary on Ginny Weasley, and you can see it
It is true that towards the end of the second book and movie, Harry Potter and the Chamber of Secrets, Dumbledore insinuates that he knows Lucius planted the diary on Ginny, but he never says it.
Harry, however, says it when he confronts him, offering him the diary back, and we all know how that turns out. He tricks him into giving Dobby a sock and thus freeing him. However, Lucius never admits it.
Now, the book doesn’t ever state this, not exclusively, but the movie does show a moment when Lucius plants Tom Riddle’s diary on Ginny, and it is, as we all know, in that scene in the library, during Gilderoy Lockhart’s book signing.
Now, what do we know at this point?
If you read the books, by the time the second movie comes out, we are already at book four and Harry Potter and the Order of the Phoenix is being written. Now, undoubtedly, this wouldn’t be the only time J. K. Rowling participated in putting some small, clever details in movies that are connected to the book’s current timeline.
We do know that Lucius was a deatheater, that much is clear from the minute he is introduced, which is right here in this scene in the library, down at Diagon Alley. He is Draco’s father, an ex-Death Eater i.e. a member of the most elite inner circle of Voldemort’s followers, and an extraordinarily rich man.
But he is keeping up appearances that he has renounced his old ways and is now an ordinary wizard. I say appearances, because he obviously knows that Tom Riddle’s diary is a Horcrux and that it can bring the dark lord back. And it does so at the cost of the host’s life, which he knows will be Ginny, so Lucius Malfoy is nothing short of a murderer!
How does he do it?
He insults everyone around, Harry, Hermione, and the entire Weasley family, including Arthur Weasley, and all that just to create a distraction so he can plant the diary.
Lucius starts by inspecting Harry’s scar, very rudely, calling Harry foolish for speaking Voldemort’s name. Second, he calls out Hermione for having muggle parents and, lastly, he begins with the Weasley family.
He turns to Ron and the others and says:
“Let me see… Red hair, vacant expressions…tatty, secondhand book. You must be the Weasleys.”
While pointing out that Ginny’s book is tatty, he picks it up from a container she kept it in. He even faces off Arthur Weasley, calling him a disgrace for a wizard… And then, after exchanging a few more insults and hard glances with Arthur, he returns the book.
But, this time, it is not just Ginny’s book he is returning. There is a second, black, thinner book seen behind the original book Ginny had. And, it is Tom Riddle’s diary, hiding in plain sight.