Not the best, not the worst. Better than several of the sub-par third season episodes, but could have been much better.
The premise isn't bad - a fake alien threat - although it seems out of place for an espionage show like UNCLE. Also echos a similar premise from "The Wild, Wild West" called "The Night of the Flying Pie Plate". (Seems that these spy shows would steal plots from each other).
Both Solo and Illya "get the girl" - their own, with Solo getting Whitney Blake, late of the TV sitcom "Hazel" and Illya getting none other than Nancy Sinatra, daughter of Frank. Nancy was quite popular in the mid-sixties, with a handful of hit songs on her own and with her famous dad - and even landing the title song to the fifth James Bond movie, "You Only Live Twice".
Unfortunately, her character's pairing with Illya, whether they have chemistry or not, seems to suck the life out of the episode, in that their scenes bog down the action, for the most part.
Net-net, while this was a passable episode, it was yet another sign that UNCLE was on the slippery slope towards a cancellation that would occur mid-way through the fourth season.