CTF yet again

They took the serum and went to sleep together.
Day one, they were just joined at the hip.
Day two, she shrunk and slowly lost her legs.
Day three, her her torso became all round and soft and throbbing.
Day four, her face became all smooth and featureless.
Day five, her eyes disappeared, her arms turned soft.
Day six, he helped her stroke herself as her arms disappeared.
Day seven, she couldn't think clearly but couldn't stop cuming.
Day eight, he could relax, she was now limp and calm.
Day nine, he was horny again and asked her sister out.