More or less from the beginning, Star Trek: The Next Generation wisely limited references to the original series. Rather than overwhelm viewers with forced connections to the older show, TNG made such rare moments special enough to count. Thus, fans were delighted when, in 1991, Ambassador Spock appeared in the two-part episode Unification, teaming up with Captain Jean-Luc Picard in an espionage adventure set on the Romulan home world of Romulus.