We study the geometry of unitary Shimura varieties without assuming the
existence of an ordinary locus. We prove, by a simple argument, the existence
of canonical subgroups on a strict neighborhood of the $\mu$-ordinary locus
(with an explicit bound). We then define the overconvergent modular forms (of
classical weight), as well as the relevant Hecke operators. Finally, we show
how an analytic continuation argument can be adapted to this case to prove a
classicality theorem, namely that an overconvergent modular form which is an
eigenform for the Hecke operators is classical under certain assumptions.