It's arc. An arch is an architectural element. A storyline follows an arc. An arc is a segment of a curve - like a rainbow, or a bend in a road. An arch often has the shape of an arc, but unless it's cut into a wall or holding up a viaduct, it's an arc.
Granted, the root's the same, and don't take it personally - you're not the only one who's used this recently.
Why yes, my father was an English major.