  Current detectors for Very-High-Energy $\gamma$-ray astrophysics are either
pointing instruments with a small field of view (Cherenkov telescopes), or
large field-of-view instruments with relatively large energy thresholds
(extensive air shower detectors). In this article, we propose a new hybrid
extensive air shower detector sensitive in an energy region starting from about
100 GeV. The detector combines a small water-Cherenkov detector, able to
provide a calorimetric measurement of shower particles at ground, with
resistive plate chambers which contribute significantly to the accurate shower
geometry reconstruction. A full simulation of this detector concept shows that
it is able to reach better sensitivity than any previous gamma-ray wide
field-of-view experiment in the sub-TeV energy region. It is expected to detect
with a $5\sigma$ significance a source fainter than the Crab Nebula in one year
at $100\,$GeV and, above $1\,$TeV a source as faint as 10\% of it. As such,
this instrument is suited to detect transient phenomena making it a very
powerful tool to trigger observations of variable sources and to detect
transients coupled to gravitational waves and gamma-ray bursts.
