The Helmacrons are an extremely violent and egomaniacal alien species that has decided to invade Earth and subject all citizens to their will, destroying the rest. They are also 1/16th of an inch tall. Cassie finds their spaceship and thinks it’s a toy, so she donates it to Goodwill. The Helmacrons are very angered by this, and call for help. The result is two tiny spaceships with functioning weapons, and beings who have no sense of proportion attacking the Animorphs. This is a joke, until the Helmacrons start using the Escafil device (the blue box that gives morphing powers, which Cassie has hidden under an old pump at her farm) to shrink the Animorphs down to their size.
The combo here weirdly works. The aliens are so over the top and ridiculous, and so very, very small (and unaware of what this means for them), you can’t take them seriously. They never stop being ridiculous, even as they become actually threatening, though I think it works because the bigger threat in having tiny Animorphs is more the real world and its small horrors (almost being stepped on, how long it takes to get anywhere, hair mites on a scalp, giant pores, being so tiny you can see individual cells, seeing a wolf spider from up close) than it is the Helmacrons, who end up being pretty easily defeated.
I would like this installment more if the ending hadn’t just kind of fizzled out. How did they get the Helmacrons to stand down and agree to leave? It’s just handwaved over. This book ends up just being a throwaway humorous installment that contributes nothing to anyone’s arcs, or the main arc of the story. Just a bit of ridiculous nonsense.