A non-holonomic robot travels with a constant speed in an unknown planar scene populated with arbitrarily shaped obstacles. There is an unknown scalar field in the plane. The robot measures only the (minimum) distance to the obstacles and the field value. We present a novel navigation law that drives the robot through the obstacles-free part of the plane to the curve (isoline) where the field assumes a pre-specified value, and then ensures tracking of the obstacles-free part of the isoline. This law does not attempt on estimation of the field gradient and is non-demanding with respect to computation and motion, exhibiting a fairly regular behavior. The performance of this law is justified by a rigorous global convergence result and is confirmed by computer simulations.