  The massive harmonic explorer is a model of random discrete path on the
hexagonal lattice that was proposed by Makarov and Smirnov as a massive
perturbation of the harmonic explorer. They argued that the scaling limit of
the massive harmonic explorer in a bounded domain is a massive version of
chordal SLE$_4$, called massive SLE$_4$, which is conformally covariant and
absolutely continuous with respect to chordal SLE$_4$. In this paper, we
provide a full and rigorous proof of this statement. Moreover, we show that a
massive SLE$_4$ curve can be coupled with a massive Gaussian free field as its
level line, when the field has appropriate boundary conditions.
