  Standard binary evolutionary models predict a significant population of core
helium-burning stars that lost their hydrogen-rich envelope after mass transfer
via Roche-lobe overflow. However, there is a scarcity of observations of such
stripped stars in the intermediate mass regime (~1.5 - 8$ M_{\odot}$), which
are thought to be prominent progenitors of SN Ib/c. Especially at low
metallicity, a significant fraction of these stars is expected to be only
partially stripped, retaining a significant amount of hydrogen on their
surfaces. For the first time, we discovered a partially stripped massive star
in a binary with a Be-type companion located in the Small Magellanic Cloud
(SMC) using a detailed spectroscopic analysis. The stripped-star nature of the
primary is revealed by the extreme CNO abundance pattern and very high
luminosity-to-mass ratio, which suggest that the primary is likely
shell-hydrogen burning. Our target SMCSGS-FS 69 is the most luminous and most
massive system among the known stripped star + Be binaries, with Mstripped ~3$
M_{\odot}$ and MBe ~17$ M_{\odot}$. Binary evolutionary tracks suggest an
initial mass of Mini $\gtrsim 12 M_{\odot}$ for the stripped star and predict
it to be in a transition phase towards a hot compact He star, which will
eventually produce a stripped-envelope supernova. Our target marks the first
representative of a so-far missing evolutionary stage in the formation pathway
of Be X-ray binaries and double neutron star mergers.
