In the episode "Thirty Days '', while disobeying direct orders in order to do what he felt was morally right, he was reduced to ensign and thrown in Voyager 's brig for a period of 30 days. About a year later, after working diligently at his duties, he regained his former rank. This occurred just before the events of "Unimatrix Zero '', during which it is revealed that he is fourth in the chain of command.
when does tom paris get his rank back
just before the events of "Unimatrix Zero ''