Harry finds an enchanted diary, owned by a former student named Tom Riddle, which shows him a flashback to fifty years before, where Riddle accused Hagrid, then a pupil, of opening the Chamber.
Harry finds an enchanted diary of opening the Chamber. An enchanted diary is owned by a former student. This former student is named Tom Riddle. Tom Riddle shows him a flashback to fifty years before, where Riddle accused Hagrid a pupil. This was then.