Pam's father is a veterinarian, so she is used to being around sick animals. But when her own pony gets sick, Pam needs her Pony Pals more than ever.
Her Pony Pals, Anna and Lulu, try to figure out what is making the pony sick. Can the Pony Pals solve the mystery? Will Lightning the pretty pony, be a pony in trouble for long?
On the first day of spring vacation, Pam is excited because she can be with her pony and the Pony Pals every day. But her excitement turns to worry when she discovers her pony, Lightning is sick. When her veterinarian father, Dr. Crandal, checks up on Lightning, he finds that she has colic - an upset stomach and intestinal problems. Pam knows horses can die from colic, and that ponies and horses can't throw up because of the way their digestive systems work, which makes it harder for them to rid of what is making them sick.
Pam and the Pony Pals decide to figure out what is making Lightning sick. Dr. Crandal says they worm her regularly, so it's not a parasite. The oats and hay she eats don't seem to be moldy, so it is not her feed. After looking through a book of poisonous plants and checking the field where Lightning resides, the Pony Pals don't find any. Lulu suggests watching Lightning for a full 24 hour to find out what she does all day, because that is what her dad, a naturalist who studies animals, will do to figure out what animals really do. The pony pals will sleep in the barn and take turns sleeping and watching Lightning.
Early in the morning of their watch, something happens for the first time. Pam notices Lightning bolt and run across to the other side of the paddock. When she catches up, she sees someone feeding Lightning from across the fence. Pam yells "Stop!" and her dog Woolie barks at the girl. When the girl notices, she drops her bag of apples and speeds away on her bike.
At first the girls think someone has been feeding Lightning poisoned apples, but her father thinks they look normal, and that Lightning has gotten sick from overeating, if she is eating three or four apples in addition to her normal diet.
Meanwhile, Anna and Lulu are excitedly preparing for the upcoming Wiggins Horse Show at Reggie Olson's barn. Pam, on the other hand, is much less enthusiastic. Despite her mom being a riding teacher, Pam has always disliked horse shows, and just wants to trail ride her pony. Nonetheless, she supports Anna and Lulu in preparing. They spend a long time grooming their ponies and they pick a Pony Pal color - periwinkle blue - for them all to wear during the horse show.
When Anna remembers that she has seen the girl with the black racing bike around town, the Pony Pals formulate a plan to talk to her. Anna has noticed the black racing bike go by her window around 7 in the morning, so fast as if it was in a race. The Pony Pals decide to wait for her on Main Street. They decide to hold up a sign that she will see, that says "STOP. MUST TALK TO YOU. VERY IMPORTANT". The girl stops and is skeptical of them at first. She says Pam is the kid who scared her the day before, and asks, "Is this some silly game?" The Pony Pals tell her ponies can't throw up, which she thinks is ridiculous. She is about to leave, but the Pony Pals stop her. When they tell her she knows about bike racing the way that they know about ponies, she finally believes them, and feels bad that she may have hurt Lightning. She promises to never feed her again.
The girls decide to visit Anna's mother diner, the Off-Main Diner, to celebrate. As they sit down to eat, their adult friend Ms. Wiggins comes in. She has presents for all three girls - periwinkle blue satin vests for their performance in the upcoming horse show.
The day before the horse show, the girls decide to ride their ponies to Olson's farm, so they will know how long it will take to get there from Anna's house. During the ride, they run into Diane, and tell her they will be in a horse show the next day. She asks how Lightning is doing, and tells them she will be in a big bike race in Virginia next weekend. Diane promises to go to the horse show. Pam declares that she and Lightning won't be in the horse show, because they don't like horse shows.
When they arrive at the Olson farm, it is a busy place, with people setting up jumps, unloading a barbecue pit, and putting up judges' stands. Pam feels depressed and left out because her friends will be in the horse show, but she doesn't want to be.
Later in Anna's room, they discuss the only horse show Pam was in before - when she was much younger and had chicken pox. She did badly, and felt embarrassed about it because she was the riding instructor's daughter. Pam is tired of everyone nagging her about the horse show, but agrees to be in it when Lulu suggests she enter the Pleasure Division where all she has to do is what she would normally do on a trail ride, like opening and closing gates and jumping over logs. Pam also has an idea about talking to Diane again.
The next morning, very early before the horse show, the Pony Pals go again to Main Street where they know they will see Diane riding. Sure enough, she shows up and smiles when she spots the Pony Pals's signs, which read "DON'T STOP!", "KEEP RIDING!", and "LIGHTNING IN HORSE SHOW!".
The Pony Pals ride back to Anna's paddock, and Lulu's Grandmother Sanders shows up to help them braid periwinkle-blue yarn into their ponies' manes. Pam starts to have fun and feel a bit better about horse shows.
At the show, they look at their schedule of classes. Lulu is in the Pony Hunter classes, Anna is in the Short Stirrup classes, and Pam is in the Open Pleasure Pony classes. Anna wins second place, Lulu wins first place, and Pam wins second place in their respective classes. Diane congratulates them on their ribbons. After the last ribbons and championship cups are awarded, Mr. Olson decides there will be a pony and horse parade, and Ms. Wiggins invites the three Pony Pals to ride right behind her pony cart, which is at the front of the parade. Before the girls dismount, a reporter for the County Times takes a picture of the three of them on their horses. Pam feels much happier about her decision to participate in horse shows.