A mighty alien empire sends their greatest, most fearsome warship on a mission to conquer the planet Earth... with one hitch. Misjudging the dimensions of the planet, when the (tiny) aliens land in and English pond, they take the frogs (and other amphibians) that live there to be indicative of life on the planet, and our only hope at that. Soon, the two sides are engaged in a solid alien-vs-amphibian battle for the sake of Earth, or at least, someone's backyard in England. It sounds a bit like a cross between Wind in the Willows and Micronauts.