The Bittering family lands on Mars because where they lived on Earth there was a Atomic war and atomic bombs so they came to Mars to get away from it so they wouldn't get hurt.
Harry wants to go back to Earth because he thinks it's not right to be on Mars because they are from Earth and not from Mars. They also get the news that all the rockets on Earth got blown up from the Atomic war so they were stuck on Mars.
What? We just got here.
This isn't right, we need to go back home.
Everything starts changing like the color of their eyes, their food isn't the same food as what they were growing, their language, etc.
Harry comes up with the idea of building a rocket ship back to Earth because all the rockets got blown up on Earth by the atomic bombs and Harry asks other people on Mars to help him build a rocket but they just joked around with him.
Really! It's going to take 30 years to build.
The Bittering family turns into Martians and they start living like Martians, speaking like Martians, eating like Martians, relaxing like Martians, etc.
We are Martians!
Five years pass and people from Earth come looking for the people on Mars but they don't see anyone in sight but Martians. Little did they know that they were going to meet the same fate as the last batch of humans.