  We present a toy model for five-dimensional heterotic M-theory where bulk
three-branes, originating in 11 dimensions from M five-branes, are modelled as
kink solutions of a bulk scalar field theory. It is shown that the vacua of
this defect model correspond to a class of topologically distinct M-theory
compactifications. Topology change can then be analysed by studying the time
evolution of the defect model. In the context of a four-dimensional effective
theory, we study in detail the simplest such process, that is the time
evolution of a kink and its collision with a boundary. We find that the kink is
generically absorbed by the boundary thereby changing the boundary charge. This
opens up the possibility of exploring the relation between more complicated
defect configurations and the topology of brane-world models.
