Big Bug is the surname of Qaroon; the richest and the most powerful mouse in the world that has a personality like a godfather. In the beginning of this fantastic, funny, musical & action movie, we see that World Bank requested a loan from Qaroon's Bank, But as you well know a bank can't give a penny of loan without a valid collateral, and Mr. Qaroon (Big Bug) has agreed to pay the loan if the General Secretary of United Nations accepts the World Bank's collateral. Qaroon, Farhad, Faramarz, Shirin, Esmal and ..., are the mice and are living with humans and these mice corporate with humans to create very lovely, fantastic, musical and action movie for children as well as adults.
As a one line synopsis of the Big Bug we can say:
Money and wealth is good but happiness and kindness is better.