Mates of State: so happy, so sad, so right, so wrong. (Photo by Nick Gordon)

There is absolutely nothing wrong with “Mountaintops,” their new, time-traveling exercise in sunny retro pop. It’s flawlessly, thoughtfully made. Its harmonies are divine; its melodies (a blend of ’80s organ-based pop and airy indie pop) immaculate.

Mates of State are scheduled to perform at the 9:30 Club on Sept. 21.