When Jimmy breaks out in hives, Casper and his friends must bring him to the hospital. But when the hospital staff gets a look at Mantha and Ra, they become so busy trying to resuscitate them they pay no attention to Jimmy.
Mr. and Mrs. Bradley go to visit Jimmy's great aunt so they make Jimmy stay home with his babysitter Jennifer since they don't think Jimmy is old enough to look after himself yet and Mr. Bradley warns him that if he doesn't do everything Jennifer says or he will be "grounded into the next century". When Jimmy's parents leave, Casper, Ra and Mantha arrive but Jimmy tells them the news, Casper is disappointed since they are all going to play at Casper's uncles' manor so Jimmy takes advantage that Jennifer is too distracted speaking on the phone to tell her he is going outside and leaves for Casper's, taking a bag of cinnamon cookies Jennifer got him and eating some on the way.
In the Ghostly Trio's manor, the kids play Zombie Attack with Mantha chasing the boys around the hall until they notice Jimmy has rashes all over him and is very itchy. The kids call an ambulance but the paramedics mistake Ra for an injured human boy and end up taking him instead despite the boy's protests, Mantha tries to get him out of the ambulance with Jimmy pulling her by her arm but she ends up losing the arm and is dragged along with Ra when the ambulance leaves. Casper suggests calling Jimmy's parents but Jimmy refuses to do so since he could be grounded for leaving the house so Casper agrees to take him to the hospital and back before Jimmy's parents return and carries him to the hospital in a chair along with his cookies.
At the hospital, Ra is put on a stretcher and rushed into the building, Mantha runs after him until a nurse grabs her and places her on another stretcher, believing her to be sick and injured due to her dead-like appearance and missing arm, the kids try to tell the staff they aren't sick but the doctor believes they are delirating and says Ra is going to need new bandages since the ones he wore were filthy, much to Ra's joy. When Casper finds them and gives Mantha her missing arm, he manages to get the doctor to examine Jimmy, it turns out that Jimmy had a serious allergic reaction to something and asks him if he has been doing anything out of the ordinary, since Jimmy spent awhile playing with Casper he starts to believe he might be allergic to ghosts, another nurse then calls for the doctor to tend to Ra. Jimmy wonders what he is allergic to and Casper says it would be something at his uncles' house, Ra, Mantha or even Casper himself then he ties Jimmy up with some tape to prevent him from scratching and declares that he is calling Jimmy's parents but Jimmy pleads to him not to.
Meanwhile, the doctor tries to defibrillate Ra but it doesn't work, when he asks for new bandages Mantha scolds him reminding him that they are there for Jimmy, not bandages but when she tries to get the doctor's attention she loses her other arm and the doctor decides to operate her, Mantha decides to go along with it since she wants sutures. Casper goes to look for the doctor when he finds Ra and Mantha being carried on stretchers and tells them about Jimmy's allergy, when he comes back for Jimmy he finds that the doctor already cured him, he tells the kids the real cause of Jimmy's allergy: the cinnamon cookies, much to the kids' relief. When Jimmy and Casper try to leave, the doctor stops them and asks if there is an adult with them, they say he is in the Cafeteria but the doctor doesn't seem to believe them. In another room, a nurse tries to examinate Mantha and asks her to let her see her tongue, only for Mantha to scare her by literally removing her tongue from her mouth, once she is gone they take the chance to escape, the doctor asks Casper and Jimmy for their names and addresses until he sees Ra and Mantha trying to escape from another nurse and the boys run away as well, the doctor and several nurses chase after them until they escape in a cab.
Once they reach Jimmy's house, Casper, Ra and Mantha say goodbye to Jimmy before going back to Casper's while Jimmy goes back home right before his parents arrive. That night, Jimmy's parents go out again and leave Jimmy with Jennifer, Jimmy sits alone in his room until he realizes his friends are in the yard, Ra asks Jimmy if he could get him some bandages from a sale at Deedsmart, Jimmy declines since he promised he could stay home but brings them some bandages, strings and needles from his family's first aid kit before walking back home since Jennifer is making cinnamon buns until his friends tackle him, Jimmy smiles and the episode ends.