The story is set in the distant future, where humanity is at war with "Yuks", an alien life form which does not use mechanical spaceships nor constructions.
Instead, it relies on life forms. The war has been going on for a long time, and humanity has not been able to come up with a solution against the life-form based ships and mines that the Yuks use. A human brain-controlled spacecraft would mean mechanical perfection. This was accomplished, and something unforeseen: a strange entity called . . . Mr. Spaceship!