Opening with a great hook, "Contagion" has the Enterprise rendezvousing in the Romulan Neutral Zone with its sister ship, the Yamato (established in "Where Silence Has Lease"), only to have the Yamato suddenly explode, killing everyone aboard. A Romulan Warbird subsequently, and ominously, arrives on the scene.
What happened here? Were the Romulans responsible? And why was the Yamato in the Neutral Zone? Yamato Captain Varley's (Thalmus Rasulala) mission was an urgent archeological chase, looking for the homeworld of the legendary Iconians, an advanced society that went extinct thousands of years ago after being besieged by its many enemies. Varley ventured into the Neutral Zone to find the Iconian world and their surviving technology, lest it fall into Romulan hands. Widespread malfunctions aboard the Yamato, however, made the ship virtually inoperable and, ultimately, doomed it to its destruction. Varley said he suspected possible design flaws, which leads the Enterprise on a hunt through its systems to find its own possible problem. The answer: a computer virus infected the Yamato when it was scanned by an Iconian probe. The Enterprise itself becomes infected when it downloads the Yamato logs.
As TNG procedural tech stories go, "Contagion" is a fairly entertaining one, with its blend of ancient archeological mysteries, ominous Romulan threats (this marks their first real appearance since season one's finale), computer tech talk, and sometimes-amusing system malfunctions (the Enterprise as well as the Romulan ship become unmanageable messes). The notion of the Iconian "gateway" technology is fascinating, even though I found myself wondering how an Iconian automated launching bay manages to continue functioning (not to mention being so dust-free for the away team) rather than falling into ruin after all these centuries.
What doesn't hold up is the plot advancement surrounding the computer virus and the Enterprise's solution, which is to essentially wipe the affected hard drives and restore them from backup. In a word: Duh. Shouldn't that have been the first course of action? And doesn't the Enterprise computer have virus-protection software? I also find it doubtful that the Yamato crew wouldn't be able to figure out what was going on when they had just as much information as the Enterprise crew. But I quibble on a basically solid show.