Spoiler.....
The city is breached and they have to evacuate but they only take the young in a train to the lighthouse that they have being mentioning little mushroom is but to work in a center where they have a lot of children all the while he is looking for his spore. After a lot of searching he finds it only to discover that they have been experimenting with it and another think he finds out is that his spore likes lu feng and follows him from inside his confinement tank.
From what I remember they discovere that his spore is inmune to all virus of the insects and are planning some kind of cure with it but little mushroom is already looking for a way to get it back.
Then the city they are in is invaded as well an he takes that chance to get his spore and run to the wild trying to go back to the forest he was in the beginning....
The story is way to long to describe in the comments because we are literally in the first chapters but I really recommend you to read it because I feel that the manga does really capture the development between the too. In the novel it feel way more intense that in the manga
Spoilers pls i cant take it anymore