When two best friends Jill and Joy find a huge sum of money on the street on the first day of the summer holiday, they decide to buy a house from the mysterious Mrs. Rosebud. It’s a real dream house: measured and equipped exactly for two little seven-year-old girls’ requirements. Based on a beloved children’s book, Jill and Joy is a story of a dream summer without parents or boundaries. Anything can and will happen as reality and fantasy magically intertwine.

Jill and Joy (both 9) live in their own house like any other little girls, except that they might be a little happier than most little girls, as they get to live in their own house specifically designed for the two. One December evening a small car, barely the size of a shoebox, pulls up in front their house, and a teensy-weensy family gets out; it's the McTiny family. The family has become homeless after a road roller smashed their home. Planning to live with the girls temporarily, the family moves in with Jill and Joy but soon finds out that not all big people have good intentions towards them. Jill and Joy's Winter is based on a beloved book by the same name, written by a Finnish writer named Marjatta Kurenniemi.

A new orphanage opens near Jill and Joy's house, and a boy named Pekki runs away from there. Pekki ends up living in Jill and Joy's cabin and tells the girls how bleak life is in the orphanage under the strict command of the headmistress Minna Pinna.

Jill and Joy receive a mission from their neighbors Tingelstein and Tangelstein: the ladies send the girls to look after their absentminded inventor brother Gadgetus. Jill and Joy get to know the wonders of Gadgetus’s farm, but everything is at risk of being destroyed when Samuel Shortcut arrives on his bulldozer, ready to destroy everything so that a new highway can be built in the area. Jill and Joy save the farm with the help of a device invented by Gadgetus, but before that they have to get to know the world of dreams and find help from there. Armstrong, a miniature pig, also takes part in the adventure and helps Jill and Joy, along with the other people living in Rose Alley.