Prelude: A shadow play.
The silhouette of a human arm. Two mosquitoes appear and hover above it. One lands on the arm and plunges in her needle nose. She fills up with blood as the other mosquito hovers uncertainly. A hand appears: SPLAT! and crushes the drinking mosquito. The uncertain mosquito makes a hurried escape…

ACT I In the leaves of a birch tree, a chorus of fat caterpillars eats and sings. Caterpillar’s voice rises above the rest; he knows he’s about to go through a metamorphosis but he doesn’t know what changes that will bring. Mosquito lands near Caterpillar, looking for Paper Wasp. She’s starving, but refuses to drink blood because she believes that it’s too dangerous. She thinks Wasp can help her. The caterpillars ready themselves for metamorphosis. Only Caterpillar wants to know what he will be after this transformation; he decides to leave with Mosquito to ask Wasp what he’ll become.

Caterpillar and Mosquito land below the birch. Mosquito smells nearby prey and struggles to resist her urge to bite. She is desperate to find Wasp. Dung Beetle appears, happily adding fresh material to his ball of dung. He points them to Wasp’s house.

Wasp is lurking in his library, waiting for Caterpillar and Mosquito. He offers Mosquito a cocktail of fermented nectar, but warns her that, as a girl mosquito, she will need to drink blood eventually. Wasp tells Caterpillar that he will become a Luna moth. Caterpillar is not impressed. When Wasp tries to show Caterpillar his treasured picture of a Luna moth, he discovers that Caterpillar has eaten it. Exasperated, Wasp throws his visitors out. Outside it is twilight; Dung Beetle rolls his ball home as Wasp shuts the windows. Mosquito, drawn to the night, flies away. Caterpillar decides he is happy to stay a caterpillar.

ACT II Night on the ground. Caterpillar is alone. He collapses in despair, realizing that even if he finds his way back to the birch tree, the other caterpillars will all be gone. Suddenly, the Fireflies rush in, with a bedraggled Mosquito in tow, lighting up the night with their song and dance. As the Fireflies dance, they illuminate two enormous eyes and vanish in fright.

Caterpillar and Mosquito gradually realize that these “eyes” are actually the spots on the wings of a resting moth. Caterpillar is furious with Mosquito for leaving him like she did. Their argument is interrupted when they discover Luna Moth, half-slipped from her cocoon. Her wing is trapped and she is keening softly. Caterpillar helps to free her. As her majestic wings unfold, so does her song. Caterpillar is mesmerized and dreamily watches her float away.

Mosquito realizes that this is the moth that Caterpillar will become. She offers to help him get home before it's too late. Suddenly, however, she notices that they are near Wasp's house. She decides to ask him for more nectar, leaving Caterpillar alone again. Deeply disappointed in her, desperate, and unsure of his way, Caterpillar turns to go and is swallowed by the ground.

Caterpillar has fallen down a dark hole. Spider emerges from the shadows. She is about to eat him when there’s a knock at the door. Spider shoulders her pet Shrew aside and goes to see who the intruder is. Mosquito slips in, unnoticed, and try to free her friend. Spider returns with Paper Wasp. They notice Mosquito, and laugh at her anemic rescue attempts. Overhead, Dung Beetle is heard singing his work song. In a flash of inspiration, Mosquito realizes that she must drink blood to have the strength to rescue her friend. She plunges her needle nose into Shrew’s backside and takes a long drink of his blood. Shrew shrieks and jerks away from the door, pinning Spider and Wasp. Dung Beetle helps Mosquito hoist Caterpillar out of the hole.

In the birch leaf litter on the ground above, Mosquito feels deliciously full and triumphant. Caterpillar examines the peculiar brown sacks among the dead leaves. A ray of light illuminates the sacks, revealing the outlines of caterpillars dancing inside; they are spinning their cocoons. Caterpillar is jubilant. He is not too late! Waving goodbye to Mosquito, he climbs into a cocoon of his own. The End