女の子とバスケットボールで勝負 (1vs1のミニゲーム…、的な？)
負けると服を脱ぐルール。数回負けた後のため、すでに下着姿です。

This girl has been challeged to a basketball game (A 1v1 minigame...?)
The rules are that the one who loses needs to strip an article of clothing. After losing a few times, she's down to her underwear.