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"I shot J.R" was part of an advertising campaign promoting the television drama Dallas, which aired on CBS from 1978-1991. In the beginning it was centered around the Ewing family, specifically the marriage of J.R. to Pamela Barnes, daughter of a rival family. Eventually the series began to focus on J.R's corrupt business dealings, becoming a trademark of the show.
The season finale of the 3rd season was titled "A House Divided". After hearing a noise outside his office, J.R. goes to investigate and is shot by a mystery assailant, ending the episode. This is one of the highest watched episodes of television of all time. The identity of the shooter would not be revealed until after the summer hiatus.
During the eight month break, many advertising campaigns were launched, including things such as "Who shot J.R.?", and during the 1980 election "A Democrat shot J.R." amongst others. The success of this episode is credited with starting the trend of television shows to end their seasons with a cliff-hanger.