Tom Paris teases Torres about a good-looking ensign who's crushing on her. This is actually a veiled attempt by Paris to ask her out himself. B'Elanna doesn't get the chance to refuse, because along come a pair of hideous cock-blockers. Giant face-challenged insectoids who look like burnt meat.
The gabbling alien meanies have a language the Universal Translator can't figure out. Neelix doesn't know their name, numbers, or culture, but they do kill trespassers. And they're probably not just screaming "RAID!?!" Bypassing their space would cost Voyager 15 months... but Janeway's been looking for somebody new to antagonize now that the Kazon are behind them. Time to blunder blindly through someone's home and see if they mind! Also maybe poke them if they're peacefully asleep.
They hear the dying words of a guy from Mislen slain along with his crew by the bugs Janeway has insisted they fly through. Dozens of tiny rampaging trilobite ships latch onto Voyager and leech it. So they swat them with phasers. Done and done!
Meanwhile, over in a much more fascinating story, the Emergency Medical Hologram argues with a diva soprano from the 22nd Century recreated as his opera singing partner on the holodeck... until he forgets the lyrics to Puccini. Soon the holo-physician is forgetting where he left his instrument. And he doesn't mean his voice.
"Doctor" B'Elanna seeks out a second opinion: a holographic engineer. A re-creation of Dr. Lewis Zimmerman, the EMH's creator. The surly Diagnostic hologram points out that the EMH has loaded its personality with irrelevancies like friendships, relationships, and hobbies.
"It was only during my off hours." the Doctor explains.
"You're supposed to be OFF during your off hours!" snaps the engineer.
Kes badgers The Diagnostic Tool to find a way to save the EMH. Kes suggests grafting their adaptive heuristic matrices together. It will leave them without a diagnostic program, but give the Doctor more memory.
"The Swarm" is a personal favorite, despite the so-so battle with the space bugs of the week, because double your Bob Picardo is double your fun. Another noble suicide, if you're still counting, from the Diagnostic Tool. He's a tool. A tool. A tool, tool, tool. But he gave his life for the welfare of others, and that's not nothing.