The Lion of Mars is set in the future in 2091 on Mars, miles away from Earth. The story's protagonist is 11-year-old Bell, taken there as an infant alongside other kids. In the American colony, there are several children and adults, and other colonies surround them. Unlike Earth, sandy storms, terrestrial rovers, and algae food characterize the settlement on Mars. The people living on Mars largely rely on food and medical supplies from Earth. In most cases, the supplies are inadequate.
The teens on planet Earth do not know any other life outside their settlement because they were taken there while infants. Bell spends most of his time playing games with other teens and aiding Phinneus in tending his algae-based food farm. Bell is the youngest of the children in the settlement, but he is the most curious.
The settlement where Bell lives is under Commander Sai, who gives six strict rules every colony member must follow, and whoever breaks the law is punished severely. The first rule is that no one is supposed to go outside without a friend, and people must walk in groups. Secondly, in an emergency, an individual is supposed to ring the Bell to alert others. Thirdly, every colony member must carry a glowing stick in his or her pocket. Fourthly, no child should use rovers. The fifth rule is that no member should go beyond the American flag. The last and most important rule is that the colony members should not interact with the inhabitants of other colonies. Unfortunately, Commander Sai gives these rules without providing any explanation or justification, and this increases Bell's curiosity.
One day, Bell, out of inquisitiveness, influences his fellow teens to break the law of interaction. They decide to use rovers to ride to the neighboring colonies to explore what they are not allowed to see. However, Bell and his friends are caught and reprimanded for breaking the law. They are later released and cautioned never to repeat that mistake.
Towards the end, the most unfortunate incident happens when a scary and mysterious virus attacks all the adults in the American colony. All the adults fall sick, and their lives are in danger. The teens take over the jobs and other operations in the colony and take care of the adults. Nevertheless, the situation worsens because help from planet Earth takes too long to arrive, and Bell is caught in the middle of this dilemma. Bell enters the restricted Red Planet to discover its mysteries and look for ways of helping his people.
