Michael "Stone" Cates met and aided Karen Wexler at a party after someone slipped ecstasy into her drink. Stone was separated from his brother and sister when he was young. His mother and father died and his brother was left to take care of his siblings. Soon, his brother had to give Stone and his sister up. Stone was the younger brother of John "Jagger" Cates, who was dating Karen. Karen didn't feel like she was good enough for Jagger, so she ended it between them and Brenda Barrett comforted him.
Stone was living with mobster and owner of the Paradise Lounge, Sonny Corinthos. Sonny started seeing the young Karen, who was stripping at his club, and got her hooked on drugs. However, she hid that part of her life from everyone else. Jagger was searching for his brother with the help of Brenda, and soon enough, the two brothers were reunited. They began rebuilding the close relationship they had as kids. Jagger became a cop and got engaged to Karen. He saved Stone from a whole load of trouble when Stone was going to be the driver of a getaway car that would bust mobster Frank Smith out of prison. Jagger was later injured in a motorcycle accident, but recovered and married Karen. They rode out of town on their wedding day on his motorcycle, and headed for Chicago.
Stone began dating Robin Scorpio despite her uncle Mac's warnings for them to stay away from each other. When Stone became very sick with the flu, Robin took care of him. After Stone assured Robin that he had been tested and was HIV-negative, the two made love. After Robin told Mac what had happened, Mac was furious with both Robin and Stone. But he also decided that he needed to let Robin grow up.
Stone still had the flu and could not get rid of it. He was then tested for HIV again and the test came out positive. Stone was devastated and couldn't bring himself to tell Robin.
When a hit was put out on Sonny, Luke, Brenda, Robin, and Stone, Stone was shot and his blood got on Robin's hands. He tried to wipe it off then he ran off. Robin found him in a motel, the same motel where the two had first made love. Stone then told Robin the terrible news that he had tested HIV-positive, because one of his previous girlfriends had been a drug addict. Dr. Alan Quartermaine ordered both Robin and Stone tested. Robin tested negative, but Stone's test revealed he had AIDS. Alan started Stone on drug therapy. Afraid of infecting Robin, Stone refused to sleep with her again. When Mac found out that Stone had AIDS, he was furious, and afraid for Robin. But he came to care for Stone. Robin then got very sick with the flu and tested HIV-positive. Brenda, Robin's best friend, took the news hard and broke down.
As Stone continued to grow more ill, his eyesight started to fail. But the day of his death, he told Robin to go stand near the light of the window. His eyes slowly came into focus, and Stone saw Robin one last time before he died. Sonny was devastated, since Stone was like his little brother. Brenda also broke down again and was comforted by Sonny. Stone's memorial was held at Sonny's penthouse. Later, Robin, Sonny, and Brenda had Stone cremated and they dropped his ashes off the bridge into the water. Robin then remembered when Stone went bungee jumping off the same bridge.