Superman II perishes after visiting Riggor, "a 'leper colony' planet," and contacting "a super virus" which dissolves "his body into primitive protoplasm." Because the scientists of the National Research Institute possess records of the original's "cytoplasmic structure," they are able to create yet another new duplicate out of "artificial protoplasm," known as Superman III.
Superman learns of these events when he is accidentally transported by historians to the 24th century of a parallel Earth along with parallel versions of George Washington, Abraham Lincoln, and George Custer (Act No. 399/1, Apr 1971: "Superman, You're Dead ...Dead ...Dead!").