The "interesting story" you're referring to is likely the episode " Cradle to Grave " (Season 5, Episode 5), which is widely considered one of the series' most disturbing and memorable "unsub" (unknown subject) cases.
The BAU (Behavioral Analysis Unit) travels to Albuquerque, New Mexico, to investigate a series of young women who have been found dead shortly after giving birth. The victims are consistently troubled, blonde teenagers who have been missing for years. The investigation reveals a chilling pattern: [S5E5] Cradle to Grave
The team initially suspects a black-market adoption ring. However, the profile shifts when they realize the killers are a married couple, Robert and Linda Reinman, who lost their own baby, Michael, due to a medical tragedy. The "interesting story" you're referring to is likely
Subplot-wise, this episode features Chief Hotchner being unusually hard on Derek Morgan, a move later revealed to be a test to see if Morgan is ready to lead the team. The investigation reveals a chilling pattern: The team