We consider the Neumann Poincare operator on domains generated by two
touching disks. There can be two types of such domains, each of which has a
cusp point at the touching point of two circles. For each domain we define a
Hilbert space on which the Neumann Poincare operator is self-adjoint and
continuous. Then we computed the complete spectral resolution of the operator.
The Neumann Poincare operator has only the absolutely continuous spectrum on
the real line [-1/2,1/2]. As an application, we analyze the localized surface
plasmon resonance of a crescent-shaped domain.