  Recent work has shown that Milky Way-mass galaxies display an incredible
range of stellar halo properties, yet the origin of this diversity is unclear.
The nearby galaxy M81 $-$ currently interacting with M82 and NGC 3077 $-$ sheds
unique light on this problem. We present a Subaru Hyper Suprime-Cam survey of
the resolved stellar populations around M81, revealing M81's stellar halo in
never-before-seen detail. We resolve the halo to unprecedented $V$-band
equivalent surface brightnesses of 33 mag arcsec$^{-2}$, and produce the
first-ever global stellar mass density map for a Milky Way-mass stellar halo
outside of the Local Group. Using the minor axis, we confirm M81's halo as one
of the lowest mass and metal-poorest known ($M_{\star} \simeq 1.16{\times}10^9
M_{\odot}$, [Fe/H] $\simeq {-}1.2$) $-$ indicating a relatively quiet prior
accretion history. Yet, our global halo census finds that tidally unbound
material from M82 and NGC 3077 provides a substantial infusion of metal-rich
material ($M_{\star} \simeq 5.4{\times}10^8$ $M_{\odot}$, [Fe/H] $\simeq
{-}$0.9). We further show that, following the accretion of its massive
satellite M82 (and the LMC-like NGC 3077), M81 will host one of the most
massive and metal-rich stellar halos in the nearby universe. Thus, the saga of
M81: following a passive history, M81's merger with M82 will completely
transform its halo from a low-mass, anemic halo rivaling the MW, to a
metal-rich behemoth rivaled only by systems such as M31. This dramatic
transformation indicates that the observed diversity in stellar halo properties
is primarily driven by diversity in the largest mergers these galaxies have
experienced.
