Mickey Finn was an alien cyborg created by a race referred to as the Masters (usually referred to as "Cockroaches") to act as an advance scout. His mission was to evaluate planets the technology of planets the his Master (called The Beast) intended to incorporate into their domains.
On the evening that the Master was supposed to arrive Micky Finn ended up in Callahan's Place where he, with great effort, overcame hardwired prohibitions and hinted how he could be disabled. The patrons at Callahan's Place, after becoming telepathic destroyed the Master and freed Mickey Finn.
Due to an unknown technology referred to as Finn Magic the patrons of Callahan's place are immune to lethal physical forces, rain, and radiation unless they will it otherwise.