It's a movie about the story of The Brave Little Tailor
When Prince Vain got his shirt fixed, he proclaims that he had slain two lizards in front of the tailor's house - "two in one strike". When the tailor swats seven flies at once, he sows himself a banner saying "seven in one strike", and together with his bird he starts a journey into the world to pronounce his deed.
When a giant threatens the land, the cityfolk mistake Mickey's boast of killing seven flies with one blow to be giants. He is then forced to fight the giant for real.