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.
- It is never outright stated that the Rat wants to harm the baby. However, it is implied as it sneaks into the baby's room and, at one point, jumps into the baby's crib, but is thrown out by Tramp.
- The Rat, Si & Am, and the Stray Dogs were the only Disney Villains who didn't have redemption.
- The Rat was the only villain to die at the end.
- The Rat's gender is unclear, perhaps male since he was named Herman in the early script.
- The Rat was animated by Wolfgang Reitherman who also animated Monstro, the climax's antagonist of Pinocchio.
- Although its role is small, the Rat is likely the main antagonist because it had bigger plans and was far more dangerous than anyone else.
- Though Si & Am drove the plot of the film, the Rat is the real main antagonist as he is far more dangerous than the cats and had bigger plans than anyone else.
- The Rat's eyes are yellow.
- The Rat's breed might be a Brown Rat.