A Story Arc is a construction decided upon by the writer, simply having episodes in a sequence is NOT a story arc. Also, a story arc doesn't have to be sequential to achieve its effect of telling a story. However, each piece of the story arc can be removed out of episodic sequence and placed simply in sequence with each other and the story can be told with very little loss of information. These two different types are called Sequential Story Arcs and Non-sequential Story Arcs.