Lady is the pampered female dog belonging to Jim Dear and Darling.
When her human masters bring a baby into the house, Lady feels she's being eased out; and when Darling's Aunt Sarah introduces her nasty twin Siamese cats into the fold, Lady is certain that she's no longer welcome.
The cats wreak all sorts of havoc, for which Lady is blamed. After the she is fitted with a muzzle, Lady escapes from the house, only to run across the path of the Tramp, a raffish male dog from the "wrong" side of town.
The Tramp helps Lady remove her muzzle, then takes her out on a night on the town, culminating in a romantic spaghetti dinner, courtesy of a pair of dog-loving Italian waiters.
After their idyllic evening together, Lady decides that it's her duty to protect Darling's baby from those duplicitous Siamese felines.
On her way home though, Lady runs into all sorts of trouble which ends up being an adventure of a lifetime.