Underdog (1964)

Season 2: Episode 24

Withstanding his pain, Underdog saves Sweet Polly and captures Riff Raff, recovering the Excalibur Swords. However, one of the swords is missing. After an intense search, the missing sword turns out to be behind Underdog's cape- which solves the pain mystery. Polly and the doctors then sing a rousing rendition of "Happy Birthday" to our hero... and he blows out the candles on his cake, and some of the icing lands on the caretakers' faces.