Hey! It's a theater! The Harvard Square theater is located on Church St and has 5 screens. I love the mural outside the theater, which extends above the marquee, but the best parts are down the bottom here (click to enlarge!). The theater shows first run movies, but more importantly, they show Rocky Horror Picture Show every Saturday at midnight, with live accompaniment. Go in costume, have a blast.
There's more detail about the theater's history here .