Single dads Ted and Mark take their daughters on vacation together. Amber and Stephanie notice that their lonely dads are often sporting erections, especially when playing with them in the pool. Neither girl wants her father to be lonely (or horny), so they decide to lend a hand.
When she was young, Elaina would often share the outdoor shower with her dad after a day on the beach. It was all very innocent, but seeing her father's 'penis', as she later found out it was called, left a lasting impression on her. As she got older, she wanted very much to see it again, and not just to see it...
That probably wouldn't have been how Maria would have described it as she lay on the floor, nursing her injured hands. But when she got back from the hospital and realized that she couldn't take off her own panties, possibilities did start to occur to her.