A young couple at first normal, crash on a planet. The girl is a bit shy and the guy a bit overconfident. As the days pass and learn how to survive in this strange new world, the breast of the girl start to grow and the shlong of the boy start to get bigger, but then the body frame of the girl start to get bigger and she become more muscular, while the guy get smaller turn into a twink. It's when the boy start to get sick and the girl start lactating and the only cure is for the boy to drink her milk regularly. This accelerate the change.
In the end, she end up looking like pic related (minus the cow features, and more self-conscious/embarrassed about her new look) and the boy become a big dicked shota. As they change, there is a role reversal as the girl become stronger and the boy weaker and he is super flustered about it.