There was a painting tucked away within an forgotten attic. If you looked at it, it would shimmer with a slight gleam only if for a second. Whenever someone noticed the shimmer and investigated the painting, they would find nothing amiss. Even touching the regally painted hills did nothing but leave the feeling of the oil pastels against your skin. The Grandfather who owned the house passed on, and left his House and all of his belongings to his Daughter or Son. The Son or Daughter moves into the bigger house and takes their children with them.
Exploring the attic, a Brother/Step Brother and Sister/Step Sister notice the painting, but this time, the painting is shimmering with light. Almost as if it was a portal to another world. Intrigued by this Painting, the Teenagers, One slightly older than the other decide to throw something at the light. When it disappears, they are both shocked. Soon, a plan is formed and they go into the painting.
The world inside is nothing like the painting however, as soon as they stepped inside, they found themselves within a Slave market. It will go from there. The plot I have in mind would be a D/s Or M/s plot.