  Electronic band structures in solids stem from a periodic potential
reflecting the structure of either the crystal lattice or an electronic order.
In the stoichiometric ruthenate Ca$_3$Ru$_2$O$_7$, numerous Fermi surface
sensitive probes indicate a low-temperature electronic reconstruction. Yet, the
causality and the reconstructed band structure remain unsolved. Here, we show
by angle-resolved photoemission spectroscopy, how in Ca$_3$Ru$_2$O$_7$ a
$C_2$-symmetric massive Dirac semimetal is realized through a Brillouin-zone
preserving electronic reconstruction. This Dirac semimetal emerges in a
two-stage transition upon cooling. The Dirac point and band velocities are
consistent with constraints set by quantum oscillation, thermodynamic, and
transport experiments, suggesting that the complete Fermi surface is resolved.
The reconstructed structure -- incompatible with
translational-symmetry-breaking density waves -- serves as an important test
for band structure calculations of correlated electron systems.
