According to what purports to be a memoir– “Tea and Aliens” (aka “The Nightmare Brethren”) –most aliens are not interested in invading the Earth. This would be illegal and as well as dangerous. However there are other types of alien/human transactions that can be hugely profitable. The Bureau of Alien Interactions is in charge of preventing the more destructive exchanges. These could involve advanced tech, precious metals, rare gems, or illegal drugs. All of which are readily available from one or more of the alien worlds. To protect the Earth we have a ragtag crowd of aliens who officially do not exist. This defensive force is made possible by the Ur technology; a non-physical system that is old beyond time. Moreover, it has no user manual, which may explain why it seems slightly erratic. Slightly erratic also describes the recently drafted team of human troubleshooters. They don’t understand the recruitment process that got them into this.
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Further caution: Live organisms are no longer accepted as payment. Exceptions only if such organisms are currently extinct on Earth.