Lion was annoyed, mouse was terrified and pleaded for escape, Lion was humored and decided to let the mouse off, the mouse felt thankful and gratitude for the lion who let him go, he returns his thanks by luckily saving the lion. What I do to determine the theme is look at it from an overall perspective and divide it into different parts, there's a part where he got annoyed, another where he was unexpectedly saved...
Who ever thought that a little mouse like that would ever save or return a favor to an animal like the lion, who's at the top of the food chain? It's laughable! But never underestimate, because look at what ended up happening.
It's also like a lesson to be learned; no matter how small the good deed you do, it will be returned later in life (perhaps 3 times bigger). Or... no matter how small the friend, it could end up being an important one. The mouse in the end was the one who really saved the lion, he turned out to be "the big guy " in the end. There's many ways to look at this.
