- Narrator: Cassie
Tiny aliens known as the Helmacrons invade Earth.
Tropes:
- Breather Episode: By Animorphs standards, this book is heavy on the comedy and light on the trauma.
- Didn't Think This Through: After Cassie, Marco, and Tobias are shrunk, Jake, Rachel, and Ax try confronting the Helmacrons in cockroach and wolf spider morph, respectively. They find out the hard way that Helmacron laser weapons, which can only annoy humans and Andalites, are deadly to insects and arachnids.
- Enemy Civil War: There are two Helmacron ships, the Galaxy Blaster and the Planet Crusher. Neither one likes the other, and get in each other's way often as they fight the Animorphs.
- Enemy Mine: The Helmacrons are so annoyingly dangerous that the Animorphs and the Yeerks team up just to make them shut up.
- Eye Scream: The Helmacrons are smart enough to aim for the eyes when the Animorphs come after them in bird of prey morph. Ax is the only one to lose an eye, but it's painful enough to force him to demorph.
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Fake Nationality: Ax meets Cassie's father again, this time saying that he's from the Republic of the Ivory Coast. It doesn't convince Walter, so Ax changes it to "Canadese". - Girls vs. Boys Plot: As a last laugh, Marco starts a gender war between the aggressive Helmacron females and the submissive Helmacron males.
- Hive Mind: Helmacrons don't have a "self" the way all other sapient species in this series do. Visser Three explains that if you kill one, its consciousness gets absorbed by the next body.
- Insane Troll Logic: The Helmacrons run on this. The cincher is when Cassie and Marco find out that Helmacron leaders are all ritually executed upon promotion so that they can never make a mistake.
- Logical Weakness: Trying to shrink a morpher can fail to stick if said morpher just acquires something that isn't shrunk. The Animorphs and Visser Three acquire anteaters while shrunk, and each morph to a full-sized anteater to take out the Helmacrons.
- Mexican Standoff: The climactic battle ends with one, where Cassie in anteater morph captures the Helmacrons, Visser Three in anteater morph captures Tobias, and Ax threatens the Visser with his tail. In the end, the Helmacrons agree to unshrink everyone and Visser Three lets Tobias go in exchange for being unshrunk.
- The Napoleon: The Helmacrons are a sixteenth of an inch tall, yet they think they're the masters of the universe.
- Never Speak Ill of the Dead: The Helmacrons of the Planet Crusher sing praises of the Helmacrons of the Galaxy Blaster after the Galaxy Blaster is draconed by Visser Three. Marco points out how the crews of both ships hated each other.
- Shrink Ray: Normally, the Helmacrons are merely an annoyance. Unfortunately, the morphing cube works as a power source for their shrink ray, allowing them to bring their enemies down to their level once they've charged up from it.
- Summon Bigger Fish: After realizing that the Helmacrons can track morphers, Cassie convinces them to go after Visser Three.
- Troll: The Helmacrons allow Cassie and Marco to grovel human-style. Marco just lies on his back with his arms behind his head and starts rambling about guys who are down on their luck, and can't get dates, salt for their fries, and quarters for arcade games.
