The show began with a parody of Superman’s famous opening. “Look, up in the sky, it’s a bird, it’s a plane, it’s a frog. A frog?” Then Underdog would say in his wimpy little voice and characteristic rhyming speech "Not bird, nor plane, nor even frog, it's just little 'ole me, Underdog!”
Underdog was the crime fighting alter ego of mild mannered Shoeshine Boy. He was often called into action by his girl friend, TV reporter Sweet Polly Purebred, when there was danger. When she called “Oh where, oh where has my Underdog gone?” Shoeshine Boy would slip into a phone booth and emerge as a champion of truth and justice fighting arch enemies such as Simon Bar Sinister and Riff Raff.
Occasionally Underdog would lose his power (no, not kryptonite or catnip) but he carried an energy pill in a secret compartment of his ring. I’m not sure how his drug usage would play with parents today. Oh, wait, it was probably a Flintstone vitamin. Nevermind.
The super wimpy voice was provided by Wally Cox who played the title role in the TV show “Mr. Peepers” and for years appeared on “Hollywood Squares.”
In the early 1940’s DC comics sued Fawcett comics saying that Captain Marvel was a rip off of Superman. No such suit was ever brought against Underdog in spite of the similarities.
Dogs can get away with a lot.