This children's chapter book follows a pet pig named Mercy and her owners on a chaotic outing to a drive-in movie theater.
Mr. Watson and Mrs. Watson live with their pig, Mercy, at 54 Deckawoo Drive. One Saturday afternoon, Mr. Watson tells Mrs. Watson about a movie called
When Pigs Fly playing at the Bijou Drive-In. He mentions that the Bijou serves real butter on every Bottomless Bucket of popcorn, meaning all-you-can-eat. Mercy, who loves butter, especially on hot toast, perks up at this news. The Watsons decide to go, and Mercy oinks in agreement.
As they prepare to leave, their neighbor Baby Lincoln asks about the outing. Baby wonders if the movie is a romance, while Mrs. Watson says it is basically about pigs flying. Baby's stern older sister, Eugenia Lincoln, scoffs that pigs do not fly and that the title is a figure of speech signifying the impossible. Mr. Watson invites the sisters along. Baby eagerly accepts, and Eugenia refuses but climbs into the Watsons' convertible anyway. Two more neighbors, Stella and Frank, also join the group. Each character has a different expectation for the film: Baby hopes for romance, Mrs. Watson expects inspiration, and Frank wants a happy ending.
At the Bijou Drive-In, several other characters are already settled in. Police Officer Tomilello, who has the night off, sits in the front row with his wife, enjoying a break from duty. He has a distinctive habit of asking himself questions and answering them aloud. Nearby, Animal Control Officer Francine Poulet is on a date with Alfred P. Tomkins. Francine remarks that pigs are the wiliest animals she deals with and recalls once catching a pig, a reference to a prior encounter with Mercy. Alfred responds to everything she says by calling it "fascinating."
At the concession stand, Leroy Ninker, a small man who dreams of becoming a cowboy, sells Bottomless Buckets of popcorn. He shouts enthusiastic cowboy exclamations while advertising the real butter. Between customers, he practices his lasso skills, twirling his rope and lassoing buckets and stools.
When the Watson convertible pulls up to the ticket window, the ticket seller, Beatrice Leapaleoni, takes off her glasses, puts them back on, and blinks. She asks whether that is a pig in the backseat. Mr. Watson explains that Mercy is a "porcine wonder," and Beatrice, unable to find a rule against porcine wonders, lets them through. She mutters that the whole situation is "wonky in the extreme."
Once inside the drive-in, the overwhelming smell of butter fills the air and drives Mercy wild. Mr. Watson and Frank head to the concession stand to buy popcorn. There, Mr. Watson recognizes Leroy as the man who once tried to rob the Watsons by stealing their toaster. Leroy introduces himself as a reformed thief studying to be a cowboy, and Mr. Watson accepts this cheerfully, ordering several Bottomless Buckets with extra butter.
While Mr. Watson and Frank are away, Mercy leaps out of the convertible and follows the butter smell. She tracks it to a station wagon, where a small child holds a large bucket of popcorn. Mercy sticks her head through the window and helps herself. The child screams, calling Mercy a monster. The child's father shoves Mercy away, sending popcorn flying. Mercy decides to move on.
Francine Poulet and Alfred spot Mercy trotting past their truck with her snout in the air. Francine recognizes her as the pig from Deckawoo Drive and declares that only an Animal Control Officer can handle this situation. She grabs a net from the back of her truck and sets off in pursuit. Alfred follows, calling everything "fascinating."
In the front row, Officer Tomilello hears the child's scream and asks himself whether it was a cry of distress. He concludes that an officer of the law must step in. He puts his badge around his neck, grabs his bullhorn, kisses his wife good-bye, and heads out to make the world safer.
Mercy discovers that every car at the drive-in has big buckets of buttered popcorn and open windows. She goes from car to car, helping herself. People scream, laugh, and faint as she eats her way through the lot. When Mr. Watson and Frank return to the convertible with popcorn, they find Mrs. Watson in tears. Stella announces that Mercy is missing. Mr. Watson reassures everyone, and the group sets out to search, except for Eugenia, who stays behind and calmly helps herself to the hot buttered popcorn.
The chaos escalates. Leroy hears screaming about monsters and pigs. When Francine runs past with her net, Leroy tells her a net will never catch livestock and insists she needs a lasso. Mr. Watson, Mrs. Watson, Stella, Frank, and Baby run through the drive-in shouting Mercy's name. Officer Tomilello speaks into his bullhorn, asking himself whether everyone should remain calm and answering in the affirmative. At the ticket counter, Beatrice adjusts her glasses, declares the situation "wonky in the extreme," and calls the fire department.
Mercy, meanwhile, is having a wonderful time. She loves a chase and runs faster, kicking up her heels. Francine throws her net but accidentally catches Officer Tomilello instead. Leroy lassoes Francine. Alfred observes from the sidelines, calling everything "fascinating." A fire truck arrives at the Bijou, carrying two firemen named Ned and Lorenzo. Alfred tells Ned that the pig is making people crazy. Lorenzo recognizes Mercy as the toast-loving pig from Deckawoo Drive.
Ned and Lorenzo catch up with Mercy and offer her toast with a great deal of butter. Mercy stops running and sits down. Toast sounds like an excellent idea to her, as it always does. Ned sits down beside her and puts his arm around her.
Mrs. Watson is overjoyed when they find Mercy. Mr. Watson asks how they can ever thank the firemen. Ned says he is hungry, and Lorenzo suggests toast. Mrs. Watson enthusiastically offers to make toast for everyone. The group never watches
When Pigs Fly, and Stella and Frank wonder whether it ended happily. Mrs. Watson says she cannot imagine a happier ending than all of them being together and eating toast. Baby calls it the happiest possible ending and says it is almost like a love story. Mr. Watson agrees that it is a love story, and Mrs. Watson adds that it is very inspirational, echoing the hopes each character expressed for the movie at the story's start.