Cate was conceived from an artificial insemination because her biological father Colin Barnes worked as a sperm donor. Her younger brother was also conceived this way by Barnes because her mother favored the genes. Her stepfather had no idea about this information and raised the two under the assumption that they were his kids.
She was doing a school project and wanted to show her grandmother the family tree but uncovered the truth of her conception. She subsequently got into an argument with her mother Grace and ran off, resulting in Grace falling and hitting her head, Cate briefly being suspected in attacking her.
When Grace came to, it was revealed Cate never hurt her, and Cate was eventually rescued. She revealed she met who she thought was her father and after slow dancing to his favorite song, he seduced and raped her, which she saw as sex. It was revealed the man was actually Sean Albert, a college rival of Barnes' wanting revenge by raping his donor daughters.
She was very devastated when the detectives told her the truth. After realizing she was deceived by a stranger, she spotted her stepfather when he came to pick her up and hugged him because he had always loved her like his own. Albert was subsequently tracked down and arrested for multiple charges of statutory rape. (SVU: "Father Dearest")