On the last summer that Joey and Mary Alice go to stay for a week at Grandma's, they're teenagers—Joey is 15, and Mary Alice is 13. They both feel like they're way too cool and grown up for this.
When they arrive, they notice that there are all sorts of signs up for the town's Centennial Celebration. There's going to be a parade and plenty of activities…and Grandma doesn't seem all that into it.
On top of that, the Centennial Celebration includes a return to the old ways, so they're all going to have to dress up (and party) like they're from 1835.
Joey and Mary Alice go up to the attic to rummage around and find things that they can wear for their historical getups.
When Joey and Mary Alice emerge in their new outfits, they both look surprisingly good—and Joey realizes that Mary Alice is growing up and will probably soon have boyfriends and stuff.
Then, Grandma comes in and starts to cry because the clothes they have on are her wedding dress and her late husband's wedding outfit. How nostalgic.
Mrs. Weidenbach, the banker's wife, shows up and begs Grandma to join the Centennial Celebration Committee because she's short some people to hand out programs, prizes, and keep the bathrooms clean.
Grandma declines, and as soon as Mrs. Weidenbach leaves, she tells the kids that they're going out. She puts on her gumboots, which means that they'll be doing plenty of walking.
Joey also finds a kitten inside the house, and when he asks Grandma about it, she tells him that there's a new litter living in the cobhouse. Awww.
The kids ask her where they're going, and she says they're going to find a man named Uncle Grady Griswold—who is 103 years old.
She needs to find him because Mrs. Weidenbach is all smug and convinced that her father is the one who will win the Oldest Settler prize and her nephew will win the talent show.
So, Grandma and Joey head out on their long walk with a hamper and a kitten, and Mary Alice doesn't come along. She's got her own business to tend to.
Grandma and Joey finally reach Uncle Grady Griswold's house and find that he and his wife, Mae, are still living. He sure is old and hard of hearing.
Uncle Grady Griswold tells them that he served in both the Civil War and the Mexican-American War—which is mind-boggling to Joey, who's only studied about those events in school.
Grandma asks if they can borrow Uncle Grady on Saturday, and Mae agrees. It's obvious that he'll be entering the Oldest Settler contest at the Centennial Celebration.
On the first day of the Centennial Celebration, people pour into town, and there are all sorts of events. For some reason, Joey hasn't seen Mary Alice all day; she's been awfully busy lately.
That night is the talent show, and he knows that he and Grandma will be going to that…even if they both act like it's no big deal.
At the talent show, Mrs. Weidenbach's nephew gets up to recite a dumb poem, and everyone in the crowd feels pressured to clap because they owe the bank money. Grandma finds the whole thing pretty distasteful.
Then, a waltz starts u,p and a lovely couple come gliding out onto the stage…it's Mary Alice and Ray Veech, from the garage.
They dance so beautifully that at the end, there's thunderous applause and people give them a standing ovation.
The next day is the parade. Mrs. Weidenbach's father is on a float with a sign saying that he's the oldest settler because he was born in 1845.
But Grandma, Joey, and Mary Alice are riding on a float, too. Mary Alice holds the cup for first place in the talent contest, and Uncle Grady sits on a throne in the back with a sign over him that says that he was born in 1832 and fought in the Mexican-American War.
Mrs. Weidenbach's father turns around and sees Uncle Grady, and he gets all up in arms because he says that there's no way he fought in the Mexican-American War.
The two old men start brawling, and Mrs. Weidenbach's father even pulls out his Civil War sword. The bystanders manage to separate the old men before they can cause serious harm.
At the end of the week, Grandma sees Joey and Mary Alice off when they go to the train station, and they all share the unspoken awareness that this will probably be their last year.
As they're riding home, Mary Alice peeks in the hamper that Grandma gave them and finds a kitten. She left it there for the kids to take home with them and keep as a pet.