"ER" is "Hill Street Blues" set in a hospital. "The West Wing" is "Hill Street Blues" set in the White House. "The Sopranos" is "Hill Street Blues" from the flip perspective. And "Twin Peaks" is a waterlogged "Hill Street Blues," set in the forests of Washington state.
In short, "Hill Street Blues" is the parent of plots that don't expire in 60 minutes. You want to know how it ends? Tune in next week. But then in the next episode, new plotlines spin off, to be resolved later. In addition to the multi-episode story and character arcs, "Hill Street Blues" pioneered complex, sometimes ugly, sometimes conflicted characters. You could count on the veritable Boy Scouts Poncharello and Baker to tie a bow on the bad guy at the end of every episode of "CHiPS." And on "24," you can't tell if Jack Bauer is a hero or villain, and every episode is like a chapter in an ever-tangling book.
The break between the two was "Hill Street Blues."