  We study the defocusing semilinear wave equation in
${\mathbb{R}}\times{\mathbb{R}}^2\backslash{\mathcal K}$ with the Dirichlet
boundary condition, where ${\mathcal K}$ is a star-shaped obstacle with smooth
boundary. We first show that the potential energy of the solution will decay
appropriately. Based on it, we show that the solution also pointwisely decays
to $0$. Finally, we show that the solution scatters both in energy space and
the critical Sobolev space. In general, we show that most of the conclusions
obtained in \cite{MR4395159}, which hold on ${\mathbb{R}}^{1+2}$, remain valid
on ${\mathbb{R}}\times{\mathbb{R}}^{2}\backslash{\mathcal K}$.
