import three;

size(7.5cm,0);

currentprojection=perspective(20,18,8);
currentlight=light(8,10,2);

path3 p=(-1,-1,0)--(1,-1,0)--(1,1,0)--(-1,1,0)--cycle;
draw(p);

path3 g=(1,0,0)..(0,1,1)..(-1,0,0)..(0,-1,1)..cycle;
draw(g,2bp+.8red);
draw(surface(g),paleblue);
dot(g,5bp+.8green);

draw((0,0,0)--normal(p),1bp+blue,Arrow3());