In a fascinating alternate reality, the failure of an explosion that killed Napoleon III led to a unique twist in history, averting the Franco-Prussian War through the clever diplomacy of his son. Over the span of 70 years, the scientific community slowly vanishes, halting technological progress and leaving the world grappling with rampant pollution.
Set in 1941, we meet April Franklin, the ambitious great-granddaughter of the renowned scientist Gustave Franklin, who is engrossed in her family's quest to develop a serum for everlasting life. Although her efforts haven't yielded groundbreaking results, she has managed to help her pet cat, Darwin, gain the incredible ability to speak. In the background, the inept but relentless Inspector Gaspar Pizoni is hot on their trail, hoping to seize the familial research for his own ends.
Upon discovering a hidden message from her father within a snow globe, April is horrified to find herself pursued by a menacing black cloud. At a crucial moment, she and Darwin are saved by Julius, a small-time criminal unwittingly drawn into their chaos by Pizoni's interest. This daring escapade leads April to reconnect with her grandfather, Prosper "Pops" Franklin, who has lived in hiding from Pizoni for a decade.
Reluctantly, Julius turns Pops over to Pizoni, who takes him to a secure government facility in Fort-la-Latte. Here, he discovers a mysterious machine dredged from the sea—a revelation that it is an electric-powered airplane. Just as cyborg soldiers strike Pops' home, April realizes their house can transform into a mobile bunker, aiding their escape alongside Julius and Darwin toward Fort La-Latte.
Their adventure takes an unexpected turn when they escape on the repaired plane, only to find Pizoni sneaking aboard. A film reveals the shocking truth: two sentient komodo dragons, named Rodrigue and Chimène, have enslaved the kidnapped scientists to create an intricate jungle ecosystem below Paris. The lizards' unlikely project involves launching vegetation resistant to external threats to other planets, seeking refuge from humanity's destructive legacy.
In a series of dramatic encounters, Pizoni, Pops, and April's father, Paul, become imprisoned but eventually plot an escape. At a crucial moment during the rocket's launch, Julius covertly replaces April's serum with water. Rodrigue, upon realizing he has been betrayed, rigs the rocket to self-destruct, aspiring to eliminate humanity altogether. Tensions escalate as he ultimately kills Chimène in a bid for power, only for Julius to intervene and fatally shoot Rodrigue.
In a final act of defiance, April uses her serum to fulfill Chimène's vision, while Darwin cleverly redirects the rocket into the void of space. The remaining characters—April, Julius, and the scientists, along with a begrudgingly allied Pizoni—watch in astonishment as the rocket explodes far above, signifying the end of threats from beneath.
Pizoni earns a promotion within Napoleon V’s security, heralding a new age of scientific freedom and rapid technological progress. Although April continues her quest to unlock the serum's potential for humans, her aspirations remain unrealized. Fast forward to 2001, as humanity triumphantly reaches the Moon, astronauts unexpectedly discover the still-living Darwin, while an elderly Julius carries joyful tidings to April about their shared past.
