Thee and thou

I have not read the book so based on the TV series am I the only one who is puzzled by why only the word "thee" is used and never the word "thou"? They both correspond to the "you" of today but “thou” was used in the nominative (i.e. as a subject) and "thee" in the objective (i.e. as an object). At the time people would have been using them both.