In terms of chords and melody, Hidden Agenda has complexity on par with the typical song, having near-average scores in Melodic Complexity and Chord-Bass Melody and below-average scores in Chord Complexity, Chord-Melody Tension and Chord Progression Novelty.
Hidden Agenda is written in the key of G Phrygian. According to the Theorytab database, it is the 4th most popular key among Phrygian keys and the 52nd most popular among all keys. The G Phrygian scale is similar to the G Minor scale except that its 2nd note (A♭) is a half step lower. Chord progressions in Phrygian often rely on the major chord built off of this 2nd scale degree (A♭ major) which gives the key its distinctive sound. See the G Phrygian Cheat Sheet for popular chords, chord progressions, downloadable midi files and more!