Season 6, Episode 23
When Homer is scammed by a conman (Snake), Marge takes matters into her own hands and gives chase. Her apprehending of the crook makes her feel so proud that she becomes a cop - much to Homer's initial displeasure, especially when she arrests him for illegal parking.
Bart: Wow Mom, I've never pictured you as any kind of an authority figure before.
There are references to:
Star Wars, Police Academy, and Spaceballs.
See here for more details.
Notes & Screenshots: Lord Heath