The Disappearing Favourite

Through his own carelessness, Goemon is taken prisoner at a hot spring, and subjected to torture by the assassin team of Rose and Wolf. They want to kill Lupin, because that will make the best proof of them being the best in the world. To that end, they are attempting to make Goemon, as Lupin's associate, tell them Lupin's weakness.

Synopsis excerpt via Kitsu.