The short starts with Panty cheering because the baseball team she is rooting for, called the Giants, won the game. The mailman, whom she had sex with before the short began, remembers to give Panty a letter. Panty tells Stocking to read it out loud. The letter was an invitation for Panty to throw the ceremonial first pitch of the next season, for the Giants, to which Panty agrees enthusiastically to attend.
A few months later, in the church's living room, Brief tells Stocking, who is eating Prickles, that he was told by Panty to come to the church in order to see something awesome. On TV, Panty arrives to throw the first pitch. But instead of throwing the ball, she undresses herself and sticks the baseball in her uterus. Brief screams, while his nose bleeds, that that's not a first pitch but her uterus. He then goes on saying that this is why he loves Panty.
A Ceremonial first pitch is a longstanding ritual of American baseball in which a guest of honor throws a ball to mark the end of pregame festivities and the start of the game.
The Japanese for "ceremonial first pitch" is 始球式 (しきゅうしき Shikyūshiki), a pun of the Japanese for "uterus" (子宮 しきゅうShikyū). In the dub, this was changed to Panty being invited to open up the baseball game, and she shows her "opening", or vagina.