This children's novel contains two connected stories narrated by Tom, a young boy whose life changes when his pet goldfish becomes an undead creature with hypnotic powers.
In the first story, "A Frankly Shocking Tale," Tom's older brother, Mark, has always been a bully. When Mark receives a chemistry set for his birthday, he dons the included white scientist coat and becomes what Tom calls a real-life Evil Scientist. Mark brings home a goldfish for a school science-week experiment on pollution and dumps it into an old fishbowl. Tom presses his face to the glass, and the fish appears to mouth "Help me." Mark mixes a toxic green substance and pours it into the bowl, snaps a photo for his assignment, and announces he will flush the fish later.
As soon as Mark leaves, Tom scoops the fish from the toxic water and runs to the bathroom, but it slips from his hands and lands in the toilet, where it floats belly up. Tom radios his best friend and next-door neighbor, Pradeep, using their walkie-talkies. Pradeep suggests CPR, and Tom presses a battery from his alarm clock against the fish. The jolt revives it. They drop it into the sink, where it begins swimming. Pradeep names the fish Frankie, after Frankenstein's monster. When Tom taps the sink and greets Frankie, the fish looks him in the eye and winks.
To keep Mark from noticing, the boys replace the toxic green water with clean water tinted by green food coloring. At Pradeep's house, Pradeep's three-year-old sister, Sami, spots the fish and squeals. Frankie stares at her with glowing green eyes, and Sami goes silent, adopting a blank stare. Tom connects the glowing eyes to the "Undead Gaze" from Mark's zombie comics and concludes that the toxic chemicals and the battery shock have turned Frankie into a zombie goldfish with hypnotic powers.
Tom returns home but discovers that Mark has captured Pradeep and taped his mouth shut. Mark then catches Tom and ties both boys back-to-back beneath the fishbowl of toxic water. When Sami wanders in carrying Frankie's bag, Mark blocks the fish's hypnotic stare with protective goggles and declares Frankie his zombie goldfish, planning to use the fish to hypnotize people at school. Frankie refuses to cooperate, and Mark storms out with the bag. Pradeep uses knot skills from Cub Scouts to free them, and they lure Mark out of the house with a phone call. Tom races to the bathroom, but the toilet has already been flushed. Through the window, he spots Frankie rolling his plastic bag across the garage roof, having escaped through the plumbing.
Outside, Sami, still under Frankie's hypnotic control, has rigged a booby trap. Mark slips on the back steps, bounces off a trampoline, and crashes into the sandbox. Frankie then rolls to the top of the jungle-gym slide, climbs onto Mark's skateboard, and aims it at Mark's head. Tom throws himself between them. Frankie's eyes soften, and he deliberately flips the board off the slide to avoid hitting Tom, launching himself into the air. His bag bursts on impact and he goes belly up in a watering can. Sami begins to cry, and a tear drips into the water, reviving the fish.
When their mother returns, Tom and Pradeep claim Mark bumped his head while playing with Sami, and she takes Mark to the hospital for observation. Before they leave, Frankie slaps Mark across the face with his tail. Tom declares Frankie is his goldfish now and reflects that with Frankie around, Mark will probably go back to being only "mostly evil."
The second story, "Rules the School!," takes place six months later. Pradeep's older brother, Sanj, has been expelled from his boarding school for the gifted after hacking its computer system. Sanj bullies Pradeep much as Mark bullies Tom, and Frankie adds Sanj to his grudge list. Tom decides Frankie is not safe at home, since Mark once tried to deep-freeze the fish, and brings Frankie to school in his backpack. Mark also appears at school, claiming his own science lab "blew up."
A fire alarm sounds, and when it stops, all the school's screens display "BBEDLAM," the name of a secret organization the two older brothers have formed. On a school computer, Pradeep finds the group's website: BBEDLAM stands for "Big Brothers' Euil Deeds to Little Annoying Morons," with a mission of tormenting younger siblings and achieving world domination. While they read the screen, Frankie escapes from Tom's backpack and disappears.
Pradeep's mother, Mrs. Kumar, arrives at school with Sami and Pradeep's lunch. Sami has been re-hypnotized by Frankie, who is hidden inside her lunchbox. Tom takes the lunchbox and heads for the science lab, but when he and Sami step inside, a mosquito net drops from the ceiling. Sanj reveals the entire sequence was a trap. Mark holds Frankie while Sanj records the fish's glowing green eyes through a webcam, creating a digital "zombie fish-stare hypno-virus." Sanj tests the virus on the computer-lab screens, and everyone who sees it goes into a zombie trance. Frankie escapes down a lab sink drain.
Tom cuts through the net and frees Sami. Pradeep, who ducked under his desk before seeing the virus, is unaffected. Sanj then uses Mrs. Prentice, the school principal, to announce a mandatory video on all screens, zombifying the entire school. He orders the zombies to find Tom and Frankie. When Tom encounters a zombified teacher, he holds up a shiny metal cafeteria tray and discovers that her own zombie stare, reflected back at her, stuns her.
Tom takes refuge in the cafeteria. The lunch ladies, led by a warm woman named Gladys, are the only staff unaffected because the kitchen has no screens. Frankie bursts from the kitchen tap and reunites with Tom. Tom has the lunch ladies look into Frankie's eyes, placing them under the fish's benevolent control. Armed with shiny trays, the lunch ladies stun the incoming zombies while Tom follows with Frankie to re-hypnotize them.
Tom's walkie-talkie signals an emergency from Pradeep, who has been discovered spying on Sanj. Sanj has recaptured Frankie and set a countdown to upload the virus to the Internet. Pradeep pretends to still be a zombie and drags Tom into the science lab as his "prisoner." Pradeep reveals he memorized Sanj's password and grabs the laptop, but Sanj snatches it back with only 45 seconds remaining. Tom opens his mouth and Frankie leaps out. The fish slaps Mark into the mosquito net, bounces off Sanj's boot, lands on the laptop, and thwacks keys with his tail, freezing the countdown at one second.
Frankie collapses on the keyboard but revives after the lunch ladies bring eggs and a moldy bread roll, his preferred foods. Tom and Pradeep tie Sanj and Mark in the net, and Frankie re-hypnotizes all the zombies. He then releases everyone in the assembly hall, where the boys have set up a "Lunch Lady Appreciation Assembly." The students and staff, remembering nothing, cheer for the lunch ladies. Mrs. Prentice finds Mark and Sanj tied up in the science lab and leads them to her office to face consequences. Tom concludes that Frankie is safer at home but is grateful to have a zombie goldfish to help when things get rough.