Ah My Goddess! is about a guy, Keiichi, who accidentally dials the wrong number and gets through to a Goddess Hotline. A goddess called Belldandy shows up and offers him a single wish. He wishes she would stay with him forever and so she does. Though Belldandy’s sister Skuld isn’t happy about this arrangement and also stays with Keiichi to keep an eye on him and make sure he doesn’t take advantage of Belldandy.
One day, in Episode 20, Skuld and her big sister Urd are called away, and not trusting Keiichi to be alone with Belldandy, Skuld leaves a robot to guard them. On top of that she leaves a special dessert for Keiichi that appeared to change him into a girl! I say “appeared to” since upon closer personal inspection Keiichi finds he was still male where it counted though had breast and long hair that made him look like a girl. This didn’t make sense to me since if Skuld wanted to change Keiichi into a girl so he can’t take advantage of Belldandy, then why would she leaves his most important parts unaffected?! Anyway, that’s what happened.
When his friends come round he has to disguise himself and pretend to be a girl, but they catch on to who he really is.
Belldandy makes a medicine that should change him back to normal but accidentally gets exposed to it herself which makes her ill, so Keiichi goes on a quest in his female form to get the ingredients of a medicine that would help her instead.