on a mission

1. In pursuit of some specific duty or ambition, especially that which is of particular importance, significance, or seriousness. The spies vanished from public life for months while she went on a mission to uncover who had bombed the state building. Our company is on a mission to help those less fortunate. That's why we've created a series of new volunteer programs in our local communities.

2. Fiercely determined to do or get what one wants. After I heard how much that slacker Jim makes, I was on a mission—I stormed straight into my boss's office and demanded a raise. When my boyfriend decides he wants to buy something, he turns into a man on a mission.