Spearheading the explosion of preposterous, fish-out-of-water conceits that typified the '60s sitcom, ''Mister Ed'' serves as a reminder of the subgenre's potential and its limitations. Despite its irresistible theme song and bantering equine, ''Ed'' is something of a one-trick pony: Ed meets George Burns! Ed goes undercover as a zebra! Ed becomes a beatnik! ''Green Acres'' is also built on a single-joke premise but, crucially, manages to weave an entire bizarro universe out of it. Albert stars as buttoned-down litigator Oliver Wendell Douglas, who abandons the rat race for the sleepy town of Hooterville, where he finds his neighbors to be not humble sons of the soil but a circle of eccentric manipulators. With Douglas' Job-like travails, exec producer Paul Henning took a brazenly corny premise and, wittingly or not, wrought a tale that, if it's not quite an existentialist allegory, sits more comfortably aside darker fare like ''The Prisoner'' than ''Hee Haw.''