This episode marks a superb confluence of great writing (Richard Matheson), taut, suspenseful direction (Douglas Heyes), and bravura acting (Agnes Moorehead), and a brilliant Twilight Zone twist. A lonely woman in a deserted farmhouse must defend herself against tiny attacking aliens.
Moorehead's performance is a pantomime tour-de-force, using no words whatsoever, yet managing to make us feel for this poor, put-upon woman. She fully inhabits the character, showing us not only the character's fear, but also her resolution, fury, and -- in one touching moment -- a touch of wounded vanity mixed with pain. Although her gestures are somewhat broad, her dedication to the role make these gestures natural outgrowths of the character, not clumsy pantomime. Remarkably, an actress who repeatedly proved herself so adept at coiled up repression (e.g., "The Magnificent Ambersons", "Citizen Kane") lets herself go in compelling fashion here.
Further contributing to the energy and power of this episode is Matheson's script. Matheson's script is a model of economy -- no wasted dialogue, in fact, only minimal dialogue. Matheson's strength as a writer was always his skill for efficient and effective plotting, and this episode contains only those actions necessary to drive home the story. This, combined with Heyes' marvelous use of light and editing to heighten the mood and suspense, keep the story moving at a crisp pace.
Perhaps this episode lacks the deep moral truths of other "Twilight Zone" episodes (Matheson's episodes usually did), it more than makes up for it in suspense and brilliant character work.