Frankie Stein's father is a molecular biologist who works in genetic engineering. When Frankie's older brother David steals a test tube of grey goo thrown out by their father's laboratory, Frankie makes him give her some. Frankie's blob of goo grows into a sentient creature, which Frankie and a group of friends care for while hiding it from adults.
The Monster Garden contains examples of:
- Aloof Big Brother: David treats Frankie like a stupid little kid when he's not ignoring her. Frankie thinks it's because he feels inferior to their older brothers Ben and Mike, who are both at Cambridge, and needs someone to look down on.
- The B Grade: David came in second in physics last term. He could barely look his father in the eye afterwards.
- Blackmail: Frankie threatens to tell on David for stealing the goo if he doesn't give her any.
- Blob Monster: Monnie first comes to life as an amorphous blob with two spots for eyes. It moves around and grabs things by creating small temporary tentacles. It becomes less bloblike as it grows larger and models its body plan on the humans, forming a head, limbs, hands, feet, and digits. By the time it reaches adulthood, it seems to have lost its shape-changing abilities.
- Cacophony Cover Up: While Alf is making a hutch for Monnie, Frankie plays her radio loud so her father won't hear the noise.
- Death by Childbirth: How Frankie's mother died.
- Distant Finale: As a teenager, Frankie goes to the mouth of the stream where she last saw Monnie years ago. Then Monnie climbs out of the water and walks towards her. Monnie has been living with a dolphin pod for the last few years and has now reached its adult height, which towers over Frankie. The two share a brief reunion before Monnie swims away again.
- A Dog Named "Dog": Frankie names the monster "Monnie".
- Lightning Can Do Anything: Frankie leaves her lump of goo in a saucer by an open window and goes to bed. That night there's a violent thunderstorm. The next morning, the saucer is blackened and sooty, and the lump of goo has octupled in size and started moving around the room.
- Little Stowaway: Frankie and Monnie hide under a tarp on a flatbed truck to travel to Mendicote Woods.
- Put Off Their Food: The day Monnie starts growing and moving around, Frankie is served green jello for dessert. She can't bring herself to eat any.
- Replacement Goldfish: After Monnie swims away, Alf gives Frankie two rabbits. She loves them, but they aren't the same.
- Shoo the Dog: As Monnie's height approaches one meter, it starts to shed its gelatinous coating, revealing skin that quickly dehydrates if it doesn't get to swim regularly. Frankie knows it can't stay in her garden for much longer. Just outside Mendicote Woods, some boys start throwing rocks at Monnie. By the time Frankie chases them away, Monnie is gone. Frankie runs into the woods and finds signs that Monnie has been near the mouth of the creek. She swims into the sea to look for Monnie, but accidentally goes too far from shore, loses strength, and passes out. Monnie carries her back to shore before swimming away forever.
- The Stool Pigeon: Frankie tries to avert this. She thinks her best friend Hazel Brent is too chatty and frivolous to be trusted with Monnie's secret, so instead she goes to her more serious classmate Julia Hobson and Julia's brother John for help, even though she doesn't really like Julia. She ends up telling Hazel anyway because she feels bad for excluding her. Hazel turns out to be trustworthy, but Julia tells Frankie's father everything because she thinks Monnie might be dangerous. Terrified Monnie will be taken back to the laboratory and experimented on, Frankie runs away with it to Mendicote Woods, a forest with a summer house and a creek that she visited as a young child, where she hopes Monnie will be safe.
- "Well Done, Son" Guy: Frankie's father is distant and aloof, and mostly leaves Frankie and David to be raised by the housekeeper, Mrs Drake. David thinks he's not as good in his father's eyes as Ben and Mike, and Frankie thinks her father is a chauvinist who didn't want a daughter. Both of them hope to earn his approval by conducting experiments on the goo.Frankie: I wanted to show my father that girls were as good as boys any day of the week. Better, in fact. I wanted him to notice me, not just pat me on the head and mumble and walk away. That's why I did it.
