Aliens are invading with lasers, spacecraft, and tons of superior technology. The end of the world seems to be coming, until some kid finds the aliens one weakness. Nerf guns*.
For some reason the aliens seem to die when hit by a Nerf dart*. Shortly after discovering this weakness, the remaining military grabbed every Nerf gun they could find from Walmarts and other stores and starts pushing back.
But this raised a question, why can the aliens die from Nerf darts easily, but assault rifles need to shoot them at least a dozen times?
Alien details; Aliens are humanoid, equipped with laser(for most handheld) and plasma(for larger handheld and mounted) weaponry. They wear airtight suits because they can't breathe earth air and have energy shields that protects them from human handheld weaponry(except Nerf darts for some reason).
Humans; Humans are humans. Modern tech and standard post-apocalyptic status with everyone carrying a gun or dead. Not much to say there.
*- includes off-brand Nerf guns and darts