Walter Bishop is the son of former allied spy, Doctor Robert Bischoff (Aug. 21, 1912 - Dec. 11, 1944) (Anglicized to Bishop following World War II). His father, a scientific pioneer at the University of Berlin, conducted espionage for the Allies within the Nazi government, sabotaged German research and smuggled scientific information to the Americans. Walter grew up with a love for science, and by the 1970s, became a head developer for a U.S. Government experimental research program called "Kelvin Genetics", alongside his long-time friend William Bell. He married Elizabeth Bishop in an undisclosed year, and their son, Peter Bishop was born in 1978.
Walter and his friend William Bell conducted numerous experiments in the area of fringe science, including developing highly advanced weapons for the Army. At some point during the 1970s, Walter and Bell discovered the existence of another universe, through the heightened use of LSD, among other drugs. They constructed a "trans-dimensional window", a portal through which could view the other universe, which was more technologically advanced than our own (in "Peter", the Other Side appears to have already created digital cell phones by the year 1985). They began copying the advanced technologies they saw over there, and sold them to the Army to benefit this world. They spent much time sending objects over to the Other Side, including Bell's car, and discovered that the "multiverse" requires balance. If a car goes over to the Other Side, then a car of the same mass must be sent into our universe.
In 1985, the now seven-year-old Peter contracted a rare and savage disease. Walter attempted numerous methods to cure him, ranging from trying to build a time machine to bring a researcher who specialized in these cases from the 1930s to the present, to watching his alternate self, dubbed "Walternate", work with advanced medicine to cure his own Peter in the alternate universe, who was also ill. But Walter could not save his son, and he died in his arms. Following the funeral, which Bell never attended, Walter became consumed with watching Walternate work to save the other Peter. Through the trans-dimensional window, Walter discovered that Walternate had formulated the correct cure for his son, but did not notice as he was distracted by The Observer known as September. Knowing that Walternate would not be able to cure the other Peter, Walter created the correct compound and built a doorway into the other universe, with the intentions of curing Peter there, and then returning home.
Nina Sharp, discovering Walter's intentions and their universally-destructive repercussions, went to stop Walter from opening the wormhole on Reiden Lake (the water from the lake would absorb any excess energy the doorway released). In the ensuing struggle, Walter managed to get through the wormhole, but Nina's arm was caught in-between both universes, and she needed to have a cybernetic arm put in its place. On the journey to the Other Side, the vial holding the cure was destroyed, forcing Walter to kidnap Peter, cure him back in our universe, and then return him. On the way back, they fell through an icy lake, but were rescued by September, who warned Walter that Peter could never return to the alternate universe. After he cured Peter, he and Elizabeth could not find themselves able to send Peter back, so they decided to raise him as their own.