A pretty clever episode about two highly developed incorporeal beings who take over the bodies of various crew members to observe how the humans aboard the Enterprise make decisions and respond to a deadly threat from an alien virus. However, the episode would have benefited from a bit of CGI to at least visualize them jumping from one body to the next.
Unfortunately, a lot of the tension is once again lost because it's clear at the end that neither Trip nor Hoshi will die from the virus. In fact, both are brought back from the dead as if nothing had happened. The actual message loses its significance through this resurrection: Just like with the Prime Directive, these incorporeal beings also act according to a protocol of not interfering in the development of other species. But they have now broken their own doctrine - even though the show is halfway through the fifth and final season. One of the two characters could have died to give the episode some meaning.
But in Happy End Star Trek the motto is: If you interfere in the development of other species with the best of intentions, then it's somehow okay.