After watching an all night science-fiction movie marathon, Candace is convinced that Ferb is an alien and gathers evidence to prove it. The boys admit that they are doing something for an alien but that Ferb is not one. Her relief turns to excitement and then to frustration when her plan to bust her brothers is compromised by the alien disguised as Mom. Meanwhile, Dr. Doofenshmirtz is trying to sell an evil "-inator" under the guise of it being a cleaning device.