The following story has futanari.
This chapter takes place in a version of Heaven's Feel's true end if Saber survived.
Rin Tohsaka had a bit of a problem right now. Actually she had been having a problem for the last couple of days. While she was at the Clock Tower she had been subject to a bit of a strange accident. There was some alchemist raving about an elixir he had created that would revolutionize medicine. Unfortunately in his raving he bumped into Rin and spilled it all over her. It didn't have any immediate effects so she thought she was fine and decided to leave him alone. That is of course after firing a fair number of Gandrs at him on principle however the next day, the day before she flew back home, she noticed a bit of a conundrum when she awoke. She grew a dick. She went about finding the alchemist and demanding that he reverse it. He informed her that the only way to reverse it would be a tantric (sexual) ritual between her and two other people. He offered to join her to which she responded by using one of her gems to curse him so that there would be the sensation of getting stabbed in the balls with a hot knife for a week. The gem in question had cost her a couple hundred thousand yen. It was worth it.
Having flown home and having just eaten lunch at Shirou's she was faced with a problem. Even after trying a couple of different cures none worked and so she was considering who do the ritual with. She immediately assumed Saber would help, the girl was too kind to say no, but she couldn't decide on the third participant. She wasn't sure if Shirou's puppet body would cause problems with the ritual. Of course Sakura was out the question given that they are sisters. Even if she did have a nice rack and an ass to die for. No! Bad older sister thoughts! That left Rider, who was certainly a nice sight to behold with her long flowing lilac locks and long legs. Not to mention her hypnotically beautiful eyes and voice. So it was decided. She would ask Saber for her help and hope that Rider would agree to help as well.It took a bit of fumbling around in terms of conversation before she actually got the gumption to ask Saber.
"Don't worry Rin, I am sympathetic to your plight and would be more than willing to help."
"Thank you so much… but why exactly are you especially sympathetic?" The magus really shouldn't have looked a gift horse in the mouth, but dammit she was curious. "It involves a bout of mischief started by troublemaking old man. I would rather not discuss it in detail."
Then came the task of asking for Rider's help.