An experimental runabout with tetryon plasma warp nacelles, designed by Harry Kim and Lieutenant Lasca in an alternate timeline where Kim never joined the crew of Voyager.
The USS Yellowstone exists only in the altered reality of the Voyager episode Non Sequitur. After a shuttle accident in a time stream, Harry Kim wakes in San Francisco in 2372 to find he was never posted to Voyager and has spent the months since as a design engineer at Starfleet Headquarters.
In that life, with his friend Danny Byrd serving on Voyager in his place, he and his colleague Lieutenant Lasca have designed a new class of runabout, and the prototype Yellowstone is about to be presented to the admiralty.
Suspected of being a Maquis spy and fitted with a security anklet, Kim escapes with the help of a down-at-heel Tom Paris. The two steal the Yellowstone from the orbital yards and recreate the accident. The runabout breaks up as Kim's shuttle crosses the time stream, restoring the timeline.