After one war scare too many the human has suppressed it aggressive instincts by genetic manipulation.
Make love, not war.
Now mankind is faced with an enemy so superior, so ruthless, that it is fight or be wiped out – and humans can no longer fight; they cannot even order their robots to fight.
Violence is decadent.
Only a museum exhibit from a thousand years ago – a Royal Navy submarine complete with commander and crew, their belligerent natures intact – can hope to save the planet from an enemy to whom living space is all-important and human life entirely superfluous.