As our story opens, four hungry children stand outside a bakery eyeing the cakes. No one knows who they are or what they are doing—or, at least, the baker's wife doesn't know these things because the mysterious children refuse to give her any info. After talking the baker's wife into giving them a place to sleep that night, the kids sit down for a dinner of bread and … more bread.
As it turns out, the children are brothers and sisters: Henry (age 14), Jessie (age 12), Violet (age 10), and Benny (age 5). (Benny's age is the only one explicitly stated in the book; we've taken the others from subsequent books in the series because we're fancy like that.) Their parents are dead, and the Alden children are on the run from a mean grandfather they've never met, and that's literally all we ever find out about their background. So far, the running away isn't going all that well, though: When Henry and Jessie overhear the baker's wife say she plans to put them to work and ship Benny off to the Children's Home the next day, they flee into the darkness. They spend the next few days walking all night and sleeping during the day.
After spending a night sleeping in the woods, a nasty storm drives Jessie to seek shelter. She finds an abandoned boxcar, and the kids like it so much that they decide to make it their home. They don't have much in the way of worldly possessions—just $4, a few towels, and a couple of other small items—but they manage to turn the boxcar into a sweet crib. They scavenge dishes from the dump, and Henry builds a swimming pool. They even get a dog, Watch, who joins the family after Jessie pulls a thorn out of his paw. They're like the perfect American family, assuming you're willing to overlook the whole "runaway orphan" thing.
Henry finds a job in town working for a local doctor. While he toils during the day, Jessie takes care of the shorties and plans the menu. Dr. Moore keeps Henry pretty busy. Though he's more than satisfied with Henry's work, the doctor is curious about what Henry's whole deal is. So, he secretly follows him back to the boxcar—yikes—and then he secretly returns two more times to get further info on the kids, once in the middle of the night (nearly giving Henry and Jessie heart attacks) and once while all of the kids are at his house picking cherries.
At some point along the way, Dr. Moore realizes that the children's grandfather is Mr. Alden, the neighboring town's resident billionaire. For no real reason, he decides to keep this info to himself. Meanwhile, life in the boxcar is going great until Violet gets sick. Henry seeks help from Dr. Moore, who takes them all back to his place while Violet recovers.
The morning after the kids move into Dr. Moore's house, Mr. Alden turns up. (Does he somehow intuit his grandchildren are there, or does someone—Dr. Moore?—tip him off? We never find out.) Mr. Alden knows the kids are afraid of their grandfather, so the plan is that Mr. Alden will win them over without revealing his true identity. The plan totally works, and by the time the kids figure out who Mr. A. is, they already trust him, so everything's cool. After Violet recovers, they all move into his super fancy mansion and live happily ever after with their beloved boxcar, which Mr. Alden parks in one of his gardens.