In the movie, a depressed Lady reveals to Trusty and Jock that Jim Dear and Darling have been acting strange lately. Firstly, Jim Dear wouldn't play with Lady when came home the other day, instead asked Darling if she was alright because he was worried about her being home with only Lady, whom he referred to as "that dog," much to her shock. Next, Darling wouldn't take Lady for a walk yesterday and when Lady tried to play with Darling's ball of string Darling struck her on the back.
Upon realizing why her masters are acting so strangely, Jock and Trusty assure Lady that she has done nothing wrong, and explain that Jim Dear and Darling are expecting a baby, which they say are very cuddly and cute little human creatures. Then Tramp stops by and tells Lady that with a baby coming, things will start to change for the worse for Lady. Jock insists that Tramp is wrong, and shoos him away, but before he leaves, Tramp tells Lady that when the baby moves in the dog moves out.
Over the next few months, Jim Dear and Darling prepare for the baby such as throwing a baby shower and Darling sends Jim out to get her some food such as watermelon and chop suey. Then one rainy night in April, Jim Dear calls his Aunt Sarah and lets her know that Darling has had the baby and that it's a boy. Although it is never mentioned what they named the baby, it is likely that they named him Jim Jr, after his father. One day Lady hears Darling singing to Jim Junior and finally learns what a baby is when she goes with Jim Dear into the nursery to get a closer look and instantly decides that the adorable baby cannot be a bad thing like Tramp predicted.
A few days later, Jim Dear and Darling are leaving to go on a trip. Lady thinks that they are abandoning Jim Jr., but Jim Dear assures Lady that they will back soon, and that she will be a great help to Aunt Sarah in taking care of Jim Jr. When Aunt Sarah arrives, she runs to the nursery and begins to coo at Jim Jr, but when Lady comes in, Aunt Sarah shoos her away and Jim Jr starts crying, which Aunt Sarah believes is because Lady scared him, but it is more likely that he is afraid of Aunt Sarah's shouting. Aunt Sarah then sings a lullaby to Jim Junior, which sounds horrible compared to Darling's singing.
Later on in the movie when Lady is chained up in the dog house, she sees a rat sneaking into Jim Jr's room and tries to warn Aunt Sarah but Aunt Sarah won't listen and orders Lady to be quiet. Hearing Lady's frantic barking, Tramp comes and Lady tells him about the rat in Jim Jr's room. Tramp rushes into the house and into the nursery and fights the rat, during which he accidently knocks over the crib and Jim Jr starts crying. Tramp is victorious, and Lady, having managed to break free from the dog chain, comes in and checks if Jim Jr is unharmed. Though he is, Aunt Sarah comes in and thinks that Tramp has attacked the baby. She shoos the "vicious brute" into the closet and calls the pound to come and get him. Lady tries to show Aunt Sarah the rat but she grabs Lady and locks her in the cellar. After the pound comes and takes Tramp away, Jim Dear and Darling return home and talk to Aunt Sarah about what happened. Jim Dear releases Lady from the cellar, believing that there's been a mistake and Lady shows Jim Dear, Darling, and Aunt Sarah the dead rat in Jim Jr's room, proving Tramp's innocence and Jim Dear and Lady go off to rescue Tramp.
On Christmas Eve, a slightly older Jim Jr is seen having his picture taken with Lady, Tramp, and their four puppies: Scamp, Annette, Collette, and Danielle. Scamp starts playing with Junior's pajamas, but Darling picks up Jim Jr and takes up to his room to take a nap.
Jim Jr, now old enough to crawl and speak, is first seen playing with Scamp, which ends with the puppy making a mess trying to catch a ball. When Scamp goes with Angel to see his family after he ran away, Junior shown missing Scamp and is given a family photo by Jim Dear. At the Fourth of July picnic, the toddler is overjoyed to see Scamp "return," but is surprised to see him run away again. Junior is last seen welcoming Scamp home and Angel into the family.
- He is presumably one year old.
- Jim Jr.'s appearance is similar to Michael Darling's appearance from Peter Pan.
- Jim Jr.'s great aunt is Sarah.
- Jim Jr.'s gender is a male.