The action starts in Mrs. Macdonald's farmyard. Jack, her son, is preoccupied with his dream of building a rocket. At first this leads him astray, but eventually proves he's really a winner. His sister is the only practical member of the family, a hard-working tomboy. The family is threatened with eviction by Mr. Meany, an unfair landlord. Jack is sent to sell Marigold, the cow. Meanwhile, the arrival of a spaceship containing Buzz and Beep, two spacemen on a mission to find a cow, leads to unusual complications.|Large flexible cast