Timmy relied on his older sister to do the choirs around the house. But she often neglected her responsibilities and landed Timmy to do them for her. So he got his friend to put him in contact with a colleague who comes from a line of magicians and wizards. He got a collection of possession spells with various levels of power so he could sneak up on what his sister was doing instead and walk her body home, where they would split the work, as agreed with their parents. The spells were lightened, so that mere mortals could cast them, but he had to cast one before they expire.
The spells varied from visualizing, which referred to just getting into someones body, but not controlling it, to dematerializing and possessing someone without the world even knowing of your past existence.
Timmy chose something controlling checked the exit spell so he could get back once he got home. He activated the spell, everything around him darkened, as he was extracted from his body and then jumped back. He tried a second time, but the way seamed to be blocked. His sister was having an intense body activity, that made a strong bond, not permitting Timmy to simple have a seat. He went on to a more powerful spell to break whatever was blocking him. This next spell shot him explosively out of his body, bypassing any material barrier.
Timmy dropped in his sister’s body moments before she reached full climax, drowning in pleasure for a few seconds, before coming to his senses. He quickly stormed out in her body, apologising to her partner, as soon as he realized what he had done. He would later discover that connecting to someone in such an intense moment, amplified by first experience and with such a deep spell sealed him inside, much as a giant boulder would scatter the water and he’d be covered by al the water that settled back.