Molly is a magical black cat with a stubby tail and long pointed ears.
When the vain blue cat Lorenzo mocks her for her short tail, she places a curse on Lorenzo's long fluffy one and brings it to life. When Lorenzo's attempts to kill his affectionate, ever-dancing tail keep failing, Molly reappears, extending her neck to offer him a knife to just cut it off. Lorenzo is hesitant to make that sacrifice before taking the knife and after a fight with his tail, finally cutting it off. She then reappears in the short's curtain call ending to make a bow.
The black cat that curses Lorenzo had a much more conventional look in Joe Grant's original 1940s story sketches. Director Mike Gabriel wanted to push the concept further and devised the concept of the black cat missing a tail. He also gave Molly an unhealthy lean frame with a slight belly paunch, as if to suggest a "belly full of maggots", and a more intimidating hyena-like posture. Additionally with the character being a black cat on a black background, some of Molly's animation and supernatural abilities were inspired by the Pink Elephants. 
Molly first appears when she approaches the restaurant Lorenzo is at, frightening away four alley cats. Noticing how Molly has no tail, Lorenzo tries to get rid of his tail in many dangerous ways. After numerous failed attempts, Molly then appears with a knife to which Lorenzo feels overwhelmed upon seeing this. Lorenzo manages to make a sacrifice by accepting her offer, separating Lorenzo's tail from its owner. Molly, Lorenzo, and his own tail separated dance in a tango together at the end,