  We study type IIB superstring in the pp-wave time-dependent background, which
has a singularity at $t=0$. We show that this background can provide a toy
model to study some ideas related to the stretched horizon paradigm and the
complementary principle of black holes. To this end, we construct a unitary
Bogoliubov generator which relates the asymptotically flat string Hilbert
space, defined at $t =\pm \infty$, to the finite time Hilbert space. For
asymptotically flat observers, the closed string vacuum close to the
singularity appears as a boundary state which is in fact a D-brane described in
the closed string channel. However, observers who go with the string towards to
the singularity see the original vacuum.
