We consider time-dependent photon transport in an interstellar cloud V, containing one or more clumps whose centers are randomly distributed within V. For simplicity, we assume that V occupies the slab {s: -R<s<R}. However, most of the results still hold if V is a given three-dimensional convex region. The physical system is first written as two coupled evolution problems in suitable Banach spaces. Then, the techniques of semigroup theory are used to prove existence and uniqueness of a positive solution. Finally, some approximate solutions arc discussed.