Hilary Higgenbottom is asleep in her room, when she is suddenly awakened by what seems to be an earthquake. When suddenly Bessie, Ben, and Happy burst into the room with a food cart and they sing out "Happy Mother's Day!"
The three of them happily give Hilary bed in breakfast, and Hilary, very happy with their compassion, give the three of them a hug. But Happy, seeing the heart shaped tattoo on Hilary's arm, sadly slides out of her grip and walks out with only Bessie noticing. Bessie follows her dog into the kitchen, asking what's wrong, but then she sees a picture of Happy as a puppy sleeping against his mother, who had a heart shaped birthmark. Bessie then realizes that Happy hasn't seen his mother ever since he was a puppy. And promises happy that they'll find his mother! Happy, pleased with this, licks Bessie's face in thanks.
Bessie, Ben, and Happy go all over down that afternoon, asking around the city is anyone's seen Happy's mother, a dog with a heart shaped birthmark. Unfortunately, nobody has seen her. Their final stop is at the dog pound, where they accidentally cause quite a ruckus with the dogs. Frightened, the three of them flee the pound.
The three of them return home miserably, Bessie tries to comfort Happy, but she only makes things worse unintentionally. But when they return home, they find the house is filled with dogs all waiting to be checked if they're Happy's mother!
Hours pass by, and none of the dogs were Happy's mother, the kids and their dog sit in the kitchen, when the doorbell rings. Bessie answers it, and behind the door is a dog with a heart shaped birthmark! Bessie eagerly calls Happy, who comes and sees the dog, his eyes filled with tears. And the two dogs, mother and child, share a happy reunion.
Upstairs, in the Hive, the kids and the dogs share tea (Ben, being too young, is drinking grape juice.) This is where Bessie finds out Happy's mother's name turns out to be Rose. Bessie asks how Happy got separated from her, but Rose starts to cry at the memory. Happy quickly comforts his mother.
As time goes by, Happy and Rose do all sorts of things together, including drinking from the toilet and howling at the moon. However, times goes by, and Rose convinces Happy to do all sorts of dirty things, such as stealing food and cheating. Bessie finds them in the basement one day, playing poker with other city dogs. Angered and disgusted, Bessie scolds Happy and sends him upstairs, apoligizing to Rose for Happy's behavior. In Hilary's restauraunt, Bessie continues scolding Happy, not knowing Rose is behind them. Rose mocks Bessie, which makes Happy laugh. Angered, Bessie grounds Happy and leaves. Rose them comes to Happy and hands him a treat, and whispers something in his ear. Happy becomes worried.
That night, Happy is posing as the owner of Hilary's diner, when a truck driver negociates with him on the steaks that filled the truck. Meanwhile, Rose, with Happy distracting the driver, begins to steal the steaks, but Bessie and Ben come and expose Rose. Rose flees inside, with Bessie and Ben following. Bessie explains that Happy told them Rose's plan to steal the steaks. Hilary, angered, tried to shoo Rose out, but Rose tries to flee and accidentally smashes into a bucket of water, soaking her and washing off the heart shaped birthmark, revealing that Rose was a fake! Rose is kicked out and Happy returns inside, upset. Hilary picked Happy up and comforts him. Bessie then says that Happy does have a mom, Hilary. Bessie, Ben, Hilary, and Happy then all share a group hug.
- Lil' Orphan Happy's title is a parody of "Lil' Orphan Annie".
- This episode reveals there are 25,673 dogs in San Francisco (in the cartoon)