This is a sequel. This may seem odd in a children's picture book, one even without words, but yet you do want to read Journey first.
For one thing, then you will know why on the title page there are a boy with a purple crayon or chalk or something, and a girl with a red one, on a two-person bicycle, with a purple bird flying along through a drab cityscape -- where it's starting to rain. They go under a bridge, where there's a door.
A king burst out of it. He has a yellow one, and a map, which he barely manages to give them before soldiers haul him off. And shut the door.
But our children quickly draw a red and a purple key, since it has two locks, and set forth. . .
Amazing what can be done without words.