Mitch Marchand, 12 years old, lives with his dad in Hermosa Beach, south of Los Angeles. His dad, called "Mouse," is an 80s-era rocker, whose band enjoyed just one mega-hit. Although Mouse is a little older than the usual father with a son in middle school, he and Mitch have a great relationship. Their life is fun, calm, ordinary – until Mitch stumbles across a strange, metallic spider that seems to be gathering junk from the neighborhood and making off with it.

Mitch and his dad set out to investigate, and soon are on the trail of thousands of the spiders, tracking them across L.A., into the dark labyrinth of the city storm drains. They discover that these spiders (advanced alien robots, actually) are constructing a secret base for extraterrestrial explorers who are soon to arrive. It's finally up to Mitch and his new friend, an alien shape-shifter, to destroy the base and stop a possible invasion of the Earth.