Shelly Johnson, played by Mädchen Amick, works at the Double R Diner, which is owned by Norma Jennings, a good friend of hers. Shelly dropped out of high school to marry trucker Leo Johnson, who had swept her off her feet by lavishing her with attention. As soon as they got married, it turned out that Leo just "wanted a maid he didn't have to pay for," in her words. Leo is a monstrous home dictator, wife-beater, and dangerous criminal, and he is tangibly connected to Laura Palmer's murder.
Shelly has been having an affair with Laura's boyfriend, Bobby Briggs, who, like Laura, is still in high school, and besides being a jock, he's something of a known hooligan. Although Bobby doesn't quite tread on the right side of the law, he is at least kind to Shelly, unlike Leo. The risk of the affair is great; Bobby deals drugs for Leo and knows just how dangerous Leo is.
During the show's second season, Leo lapses into a catatonic state and Shelly and Bobby openly flaunt their relationship in front of him.