Betrayed by her sister

"Okay, I'll go look." Kat said. She turned and marched out of the water, leaving her naked sister hunched over in the water. Keira watched her go. She hunched as far as she could over, hiding as much of her body in the water as she could. Both hands were pressed over her small chest (and rock hard nipples, due to the cold water) and her legs were crossed. She couldn't believe this! She was naked on a public beach! What a disaster! Why did she always have this awful luck! It wasn't fair! Her sister didn't end up naked like this! And even if she did, what did she have to be embarrassed about? Miss perfect body wouldn't be ashamed, and no one would laugh at her!

Meanwhile the audience followed Kat walking down the beach, softly chuckling to herself. Then it cuts to a flashback in their apartment the night before. Kat is taking a scissors to Keira's bikini, cutting the straps so they'll fall apart at the slightest resistance.

We return to the beach as Kat laughs to herself.

"Everything is going exactly according to plan." She grinned. Then, instead of going to the lifeguard tower she returned to her bag, and drew out the clothes she'd brought specifically for Keira to have to wear...