This is six episodes of pure fanservice disguised as plot, and it knows exactly what it's here to do. Every character gets their moment in the onsen, the logic for why they're all there barely holds together, and that's the point. You're not here for a story; you're here because you committed to Ikkitousen and want to see your favorites in as little as possible.
Ikkitousen: Dragon Destiny Sekiheki Onsen Daikessen is a set of six special episodes from 2007, bundled with the DVD and Blu-ray releases of Ikkitousen: Dragon Destiny. They carry the Ecchi genre label, so expect fanservice as part of the package. The extras are aimed at viewers who already own or follow that season.