Consider 2D surfaces. They may be plane or curved. For a 2D surface to satisfy as a plane, it must satisfy that a line joining any two points on the surface must wholly lie on the surface. All other surfaces that do not satisfy this condition are not planes.
Now, extending this analogy to 3D spaces, we could call a space as "straight" (not sure what term I should use) if a plane joining any three points in the space wholly lie within the space. This seems to be true for the 3D space that we can observe (all points on any plane that can be considered in our 3D space lies wholly within the space)
However, is it possible for a space to be not "straight"? In the sense, can we have a situation where a plane connecting 3 points on such a space, such that the plane goes through regions not lying in the space?