Ten thousand years ago, humans came to Earth as refugees, but we have since forgotten our origins.
Three hundred years ago, another alien species came to dominate. They hid in plain sight, waiting and watching. Since we had forgotten our history, our foe could hide with impunity as a spiky green ‘plant’. We didn’t know any better.
Those weren’t leaves, and those weren’t buds, and those definitely weren’t roots. When the opportunity came, the ‘plants’ would uproot themselves and take over the world.
The temperatures had risen just enough. The big cities were fighting off the ocean, the countryside was parched, and the people were starving. While they were distracted, the ‘plants’ were preparing to make their move.
Another alien had other ideas though. Those big clouds that hover overhead: they’re not really clouds. They were sent by our interplanetary allies, five thousand years ago, to protect us only in the most dire of emergencies. That emergency had finally arrived. The clouds bloated and blocked out the sun. The world was covered in blackness.
The plants continued their assault, but their fuel would only last so long. Just like real plants, they needed sunlight to survive. They raided cities and wrapped skyscrapers. But they slowed and withered before too long.
When the clouds shrivelled and let the sun through again, the cities were covered in green. The aliens had died and their bodies wrapped cities like decorative garlands. People found that they could eat them, and the crisis of food had been solved temporarily.
The darkness had cooled the earth just enough to restore the cycle of the weather and the crops began to regenerate. And most importantly, the people had realised the error of their ways and sought never again to sabotage themselves like they had done before.