Clara and Jonah and Caleb and Briar are taking a train over the mountains from Venice to Paris. The train is weighed down with people escaping a flood in their city, luggage (including stuffed bears) in hand. Will the train make it over the mountains with all those bodies and all those things? Fortunately, the children are energy smart. Putting their heads together, they search for ways to challenge the highest peaks. Along the way, they discover that no one engine or form of energy can do it all. Making it to the top (with bears) takes a pooling of ideas and resources. In a world polarized about energy choices, it's a valuable lesson for us all.

The story is based on a modified form of the Kaya Identity, which scientists (including the Intergovernmental Panel on Climate Change) use to gauge greenhouse gas emissions and evaluate ways to reduce them.