Port dependency tool (if that's what you'd call it)

[Modulok]

I'm not quite sure how to put into word what I want, so bear with me. Is
there a tool in the base system which does something along these lines:

1. Look at the makefile of a given port as far as its RUN_DEPENDS and
BUILD_DEPENDS.
2. Subtracting what I have already installed, provide me with information
about what would be fetched (and possibly installed) in an easy-to-digest
format, recursively (for all dependents of dependents ... and so on).

If not part of the base system, is there a port which offers this
functionality?

Thank ye.
-Modulok-