Several very common French verbs are followed directly by an infinitive, with no preposition linking them — a pattern that doesn't map cleanly onto English. Aller + infinitive expresses the near future (je vais partir, I'm going to leave), vouloir + infinitive expresses a wish (elle veut voyager, she wants to travel), and devoir + infinitive expresses obligation (nous devons partir, we must leave).
Because the second verb stays in the infinitive, it never changes for the subject — only the first (conjugated) verb agrees with who's speaking. Ils vont manger (they're going to eat) keeps manger unconjugated no matter what the subject is.