After he was injured in battle, he was rebuilt as a cyborg in an experiment on geass and attacked Zero again.
After that, V.V. modified him with the geass canceller, and now he's an assassin for the order coming to kill Lelouch.
I think we're caught up on that now.
Jeremiah corners Lelouch, but he has a device that will disrupt Jeremiah's cybernetics.
Jeremiah asks why Lelouch became Zero. It turns out that Jeremiah was in Lady Marianne (Lelouch's mother)'s honor guard. His first loyalty is to her, not V.V.
He joins Lelouch's side.