Answer:
   
   The Mishna contains the detailed instructions necessary for following
   the rules that were merely outlined in the Torah.
   
   Which is a subset of which? Consider that although the basic laws of
   Judaism were revealed/developed simultaneously, only the basic mitzvot
   (without instructions on how to fulfill them) were originally written
   down. Although the Mishna was written centuries later, they are both
   of equal stature. However, because the Mishna includes most the laws
   of the Torah--and presents additional information--one could say that
   for practical purposes the Torah is a subset of the Mishna. Note that
   the Mishna does not quite cover all the laws in the Torah. Omissions
   include the laws of Mezuzot and the Priestly benedictions.
   
   When one gets to the Talmud, one sees that the Mishna is a subset of
   the Talmud, as the Talmud includes practically all of the Mishna as
   well as additional information.