B'Elanna is having a bad day. In the first few hours, everything that can go wrong for her has, from a busted sonic shower to Chakotay telling her that she's to work with Seven of Nine to attempt to open a transwarp conduit. On top of all that, the day is a Klingon holiday of reflection and comparison to proper Klingon standards.
As they begin work, Voyager encounters a small group of refugees named the Caatati, whose homeworld was assimilated by the Borg. Voyager supplies them with medicine and Thorium, which runs their power systems.
Paris designed a holoprogram for B'Elanna to help her "celebrate" the Day of Honor, but she initially decides not to go through with it. After speaking with Neelix, she changes her mind, but backs out right after starting. When Paris asks her why, she blows up, and Tom leaves her to stew.
After supplying the Caatati, they begin a test of the transwarp technology, but something goes wrong immediately. B'Elanna has to eject the warp core before it breaches. She informs the bridge, ending with "Welcome to the worst day of my life."
B'Elanna and Paris take a shuttle to retrieve the core. Until they return, Voyager is going nowhere. When they find the core, the Caatati have already found it and laid claim. When they try to reclaim it, the Caatati respond and cripple the shuttle. Paris and Torres don spacesuits and beam out into space just before their shuttle explodes, without having had a chance to contact Voyager.
While waiting for the shuttle to return, Janeway summons Seven to discuss the test failure. She is suspicious that Seven may have sabotaged the test, but Seven assures her that she did not. They review the records and Seven finds the flaw, but at that moment they receive a basic signal suggesting that the shuttle has run into trouble. At the same time, a Caatati armada arrives and demands all their supplies, thorium, and Seven.
Tom and B'Elanna try to pass the time while waiting for Voyager, but they encounter drift into a space anomaly that damages both their suits and leaves them with only half an hour of air. This makes them consider their relationship.
On Voyager, the bridge crew debates their options. Seven offers to give herself up, but Janeway refuses. Seven then realizes that she still knows how to build the Caatati's thorium generators. She creates a sample unit for them, and the Caatati return the warp core and release Voyager.
With only a couple minutes of air left, B'Elanna faces herself, and admits to Tom that she loves him. Their air then becomes too thin, and she falls asleep, moments before Voyager arrives and beams them aboard.