An imagineer said there'd be a "B story." Then they introduced Chuuby, which made some think Chuuby was the "B Story", but Chuuby just adds to the singing in the opening and closing scenes.
I guessed it was Pluto chasing the basket since he's featured on the poster with the basket. There are a few scenes with Pluto and the basket, which makes me think that that was the B Story... though, a very weak one since there are too many scenes in which he doesn't appear, and when he does appear, there isn't much going on. I would have expected seeing him being more actively chased by critters wanting the basket and Pluto more actively chasing the basket.