Old man Pettson promises his cat Findus that Santa will come to them on Christmas Eve. Afterwards, of course, he regrets it. You have to keep what you promise.
Findu's wish list has disappeared. The cat is convinced that Santa has come to pick it up. Pettson, who is not as convinced, makes a plan about how he will try to build a Santa himself for Christmas Eve.
The Neighbor children Lasse and Josefina come to buy eggs from Pettson. They both talk about Santa all the time and Findus becomes spellbound. Now Pettson is really getting worried. How should he behave, so as not to disappoint the cat?
How can you build a Santa machine and at the same time pretend it will be an automatic wood thrower? Pettson is close to giving it all up. Then Findus helps, even though he himself doesn't understand a bit.
Now Pettson must have something to hide his secret invention in. Maybe Henrik and Linnea in the box store can help? But the worst of all is probably that he has to lie to Findus all the time.
How will Pettson make his mechanical elf move? Findus wants to have fun, but the old man is just out in the carpenter's shed brooding. He sure forgets everything else. Is he going to let his own cat starve to death?
Findus also wants to be an inventor and do something creative. Signhild, who wants to sell lottery tickets to the church's bazaar, gets to be his guinea pig.
Doesn't the old man notice that the cat feels abandoned? Pettson becomes more and more preoccupied with his secret invention. But now Findus has had enough.
What a lovely trip in the nice winter weather. Pettson and Findus are enjoying themselves. But it's not easy to catch a fish, and there are others who are also hungry.
Lasse and Josefina are to be with Findus when Pettson goes shopping. But the old man has hardly had time to close the door before Dagmar from the home economics school Margareta steps straight into the kitchen and begins to decide.
What does a nice gingerbread look like? Can you use anything for cake tins? There's a lot to disagree about when Pettson and Findus are going to make gingerbread together.
Lucia day will be busy, at least for some, and the evening will not be quieter. Something happens up the road and suddenly the little cottage is full of visitors.
How can you feel calm, when there are people who constantly take away the things you need, just when you need them. And then refuses to return them. No, now Pettson is tired.
When Pettson can't get his Santa machine to work, Findus hits a postman on the toboggan run. He seems to know everything - except delivering letters. What he pulls out of his large bag may be just what Pettson needs.
Now there will finally be Christmas cleaning, Pettson has decided. It is difficult if it is to be done properly. And yet Findus is not quite satisfied.
What is that cry for help in the middle of the forest? And who is it, who knows something, but does not know what he is supposed to know? It sounds like a crossword puzzle.
Soon the Santa machine is ready, but there is only one important thing missing. Pettson doesn't know where he put it. The worst thing is that Findus found it and wrapped it - as a Christmas present.
While Pettson ponders out in the carpenter's shed how to make his mechanical elf look more alive, Findus is visited in the kitchen by a traveling hairdresser.
Remember that Findus can never go to bed, so Pettson gets to try the Santa machine. It's lucky that there is so much else to do the day before the baptism.
Of course, the idea of a Santa machine is completely insane. Even Pettson thinks so, when he thinks about it. But now it's finally Christmas Eve and in a moment everything will start. What will Findus say?