  We propose that the recently observed quasi-periodic eruptions (QPEs) in
galactic nuclei are produced by unstable mass transfer due to Roche lobe
overflow of a low-mass main-sequence star in a mildly eccentric (e ~ 0.5)
orbit. We argue that the QPE emission is powered by circularization shocks, but
not directly by black hole accretion. Our model predicts the presence of a
time-steady accretion disk that is bolometrically brighter than the
time-averaged QPE luminosity, but primarily emits in the extreme-UV. This is
consistent with the quiescent soft X-ray emission detected in between the
eruptions in eROSITA QPE1, QPE2, and GSN 069. Such accretion disks have an
unusual $\nu L_\nu \propto \nu^{12/7}$ optical spectrum. The lifetime of the
bright QPE phase, 100-1000 yrs, is set by mass-loss triggered by ram-pressure
interaction between the star and the accretion disk fed by the star itself. We
show that the stellar orbits needed to explain QPEs can be efficiently created
by the Hills breakup of tight stellar binaries provided that (i) the stellar
binary orbit is tidally hardened before the breakup due to diffusive growth of
the f-mode amplitude, and (ii) the captured star's orbit decays by
gravitational wave emission without significant orbital angular momentum
diffusion (which is the case for black holes less than about a million Msun).
We conclude by discussing the implications of our model for hyper-velocity
stars, extreme mass ratio inspirals, repeating partial TDEs, and related
stellar phenomena in galactic nuclei.
