  The black hole of an active galactic nucleus is encircled by an accretion
disk. The surface density of the disk is always too low to affect the tidal
disruption of a star, but it can be high enough that a vigorous interaction
results when the debris stream returns to pericenter and punches through the
disk. Shocks excited in the disk dissipate the kinetic energy of the disk
interior to the impact point and expedite inflow toward the black hole.
Radiatively efficient disks with luminosity $\gtrsim10^{-3}$ Eddington have a
high enough surface density that the initial stream-disk interaction leads to
energy dissipation at a super-Eddington rate. Because of the rapid inflow, only
part of this dissipated energy emerges as radiation, while the rest is advected
into the black hole. Dissipation, inflow, and cooling balance to keep the
bolometric luminosity at an Eddington-level plateau whose duration is tens of
days, with an almost linear dependence on stellar mass. After the plateau, the
luminosity decreases in proportion to the disk surface density, with a
power-law index between $-3$ and $-2$ at earlier times, and possibly a steeper
index at later times.
