Original Airdate: November 3, 1965
Invaders From The Fifth Dimension, in the form of floating alien heads, search Preplanis for a humanoid brain to power their spaceship and take them home. When they capture Smith, they quickly find out that he's, by Smith's own admission, "a scoundrel, a thoroughly bad sort, hopelessly unreliable". So, Smith offers up a "small Robinson", one that will fit in the alien ship... Will.
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Glowing humanoids invade the Robinsons' colony in search of a brain to navigate their ship. Their plan is foiled when Will's worry over his parents' safety makes his brain unusable.