Walter White's chilling line "I am the one who knocks!" marked his complete transformation into alter-ego Heisenberg on Breaking Bad. It's easy to forget now but it was quite shocking to see Bryan Cranston appear on Breaking Bad when it first aired since he was best known for Malcolm In The Middle and other comic roles. The setup for the show finds Cranston's White as a chemistry teacher who receives a terminal cancer diagnosis. In order to make some fast money for his family, he teams up with former student Jesse Pinkman (Aaron Paul, The Path) to cook and sell meth.
Walt's slow transformation throughout Breaking Bad from meek family man to hardened drug lord was startling, and made all the better by the show's superb writing and acting. The series came to an end in 2013 after five seasons, but received a prequel/sequel in the form of Better Call Saul, revealing the origins of Walt's sleazy lawyer Saul Goodman (Bob Odenkirk). Breaking Bad also gave Jesse's story a proper ending in the form of Netflix movie El Camino, which featured cameos from a few of the show's major characters.
The Walter White that started Breaking Bad and the one who ended it almost felt like two different people, with the character being seduced by the power of his position. Breaking Bad season 4 is considered by fans to be one of the show's very best and detailed his struggle - and eventual victory - against Gus Fring. The sixth episode "Cornered" also features a key turning point, where its clear Walt now sees himself more as his fictional drug kingpin persona Heisenberg.
In a pivotal Breaking Bad scene where Walt's wife Skylar tries to him to convince to turn himself in, his ego is wounded by her suggestion he's in danger and could shot just answering his door. He gives her an angry rant about how big his business has become, and that's he's not the one in danger. "I am the one who knocks!" is the line he gives in response to the idea he could be killed opening his door. Skylar is quite understandably upset by this outburst - and can read between the lines that he was partly responsible for Gale Boetticher's death in season 3.
This is the scene that showed Skylar Walt was no longer an ordinary man in over his head, and it also signaled to audiences that Walt had fully embraced his inner Heisenberg. Of course, that transformation would continue until the end of Breaking Bad and would ultimately led to Walt's destruction.