Peggy is depressed that her birthday always ends up being a bust, so she vows to plan the party of the year. Hank, Bill, Boomhauer, Kahn, Luanne and the rest of the gang gather to celebrate with a disco-themed shindig on a mystery train, and everyone gets dressed in their sharpest 70s attire. Luanne ruins the mystery, and it becomes clear this is just another typical Peggy party. However, when a couple tucks away to the train's bathroom for some extracurricular activity, a new mystery unfolds and the gang tries to find out who did the deed.