  Metallic quantum criticality is frequently discussed as a source for
non-Fermi liquid behavior, but controlled theoretical treatments are scarce.
Here we identify and study a novel magnetic quantum critical point in a
two-dimensional antiferromagnet coupled to a three-dimensional environment of
conduction electrons. Using sign-problem-free quantum Monte Carlo simulations
and an effective field-theory analysis, we demonstrate that the quantum
critical point is characterized by marginal Fermi liquid behavior. In
particular, we compute the electrical resistivity for transport across the
magnetic layer, which is shown to display a linear temperature dependence at
criticality. Experimental realizations in Kondo heterostructures are discussed.
