Lady first encounters the rat while she is running happily in the backyard. She sees the rat, growls, and chases after it, causing the rodent to scatter through a hole in the fence.
The rat has a bigger antagonistic role in the climax of the film when it comes back to Jim Dear and Darling's house through the fence hole, while Lady is tied to her doghouse on a rainy night. While Lady barks at the rat, Aunt Sarah tells her to shush. Tramp hears Lady's barks and she tells him that there's a rat in the house. Tramp follows the rat to the baby's room just in time before it can harm the infant. The two engage in a fight until at one point, the rat climbs to the top of the baby's crib, about to kill the infant. Tramp chases the rat off the crib while overturning it by accident, causing the baby to cry. Finally, Tramp manages to corner the rat behind the couch and kill it.
Having been alerted by the commotion, Aunt Sarah arrives and, not seeing the rat, mistakenly believes that Lady and Tramp had attempted to harm the baby, promptly locking Lady in the basement and sending Tramp off to the pound. However, as soon as Jim Dear and Darling return, Lady shows them the body of the dead rat, clearing Tramp's name.
- In an early script, the Rat's name was Herman. Also, Lady fought it instead of Tramp.
- The Rat is the only antagonist in the film that doesn't make any sound whatsoever other than a few squeaks, while it was being attacked by Tramp.
- The Rat's gender is unclear, perhaps male since it was named Herman in the early script.
- The Rat was animated by Wolfgang Reitherman, who also animated Monstro, the climax's antagonist of Pinocchio.
- The Rat's breed might be a Brown Rat.
Characters: Lady | Tramp | Jock | Trusty | Aunt Sarah | Si and Am | Jim Dear | Darling | Jim Jr. | Tony | Joe | Policeman | Professor | The Dogcatcher | Stray Dogs | Al the Alligator | Hyena | Mr. Busy | The Rat | Peg | Bull | Toughy | Boris | Pedro | Dachsie | Scamp | Angel | Annette, Danielle and Collette | Buster | Reggie | Francois | Sparky | Mooch | Ruby | Scratchy
Songs: Bella Notte | Jock's Song | What is a Baby? | La La Lu | The Siamese Cat Song | He's a Tramp | Welcome Home | World Without Fences | Junkyard Society Rag | I Didn't Know That I Could Feel This Way | Always There