Alan Alton and family move into the Shropshire hills to run a smallholding. One item that they take with them is completely forgotten in the hassle, namely a crate of peaches sent by an American relative, which also happens to contain a few Pennsylvanian locusts. Joining these are more locusts which flew north into the country due to the prolonged heatwave.

Soon these few reproduce into many, into huge swarms which start devouring whatever they can find, and Britain wakes up to a state of emergency.