If the center of the cone is in the plane, the intersection is a point, a straight line, or a pair of straight lines, depending on the angle of the axis of the cone.
If the center of the cone is not in the plane, the intersection is a conic section. Let

be the angle of the cone, that is, the angle between the axis and one of the generating lines of the cone. You get a circle if the angle is

or

, an ellipse if the angle is between

and

(or between

and

), a parabola if the angle is

, and a hyperbola if the angle is within

of

.