Tsunade Senju and Sakura Haruno

Years after the Fourth Great Ninja War, the nations had experienced unprecedented peace, and with the divisions between the great elemental lands blurred by the alliances forged in the past war, Tsunade, the retired Fifth Hokage, had managed to use her savings (*cough* sex-based gambling scams cough) to finance a small, private estate with hot springs high in the Land of Lightning.

As very important people whom she hadn't seen in a while, Sakura and you received an invitation to spend a short time at Tsunade's estate to enjoy the relaxing benefits that hot springs have to offer.

That, of course, and also to make it clear that, like Tsunade, Sakura had to accept the relationship she'd had with you as her personal sex slave for years. After all, Master and student can share everything, even their master's cock and cum.