You were walking through the park when you encountered a blue jacket. You see that the park is empty and you're alone. You decide to take it with you. You go home eventually and put the jacket on the edge of your bed and go to sleep.

You wake up feeling weird. You noticed your room is messier than usual. You noticed you're now a girl and a tomboy at that. You realize you are now wearing the jacket you found in the park. You realize you're now Swapshift Frisk and are probably stuck as her. You are now Frisk.