  GRB 221009A ($z=0.151$) is one of the closest known long $\gamma$-ray bursts
(GRBs). Its extreme brightness across all electromagnetic wavelengths provides
an unprecedented opportunity to study a member of this still-mysterious class
of transients in exquisite detail. We present multi-wavelength observations of
this extraordinary event, spanning 15 orders of magnitude in photon energy from
radio to $\gamma$-rays. We find that the data can be partially explained by a
forward shock (FS) from a highly-collimated relativistic jet interacting with a
low-density wind-like medium. Under this model, the jet's beaming-corrected
kinetic energy ($E_K \sim 4\times10^{50}$ erg) is typical for the GRB
population. The radio and mm data provide strong limiting constraints on the FS
model, but require the presence of an additional emission component. From
equipartition arguments, we find that the radio emission is likely produced by
a small amount of mass ($\lesssim6\times10^{-7} M_\odot$) moving
relativistically ($\Gamma\gtrsim9$) with a large kinetic energy
($\gtrsim10^{49}$ erg). However, the temporal evolution of this component does
not follow prescriptions for synchrotron radiation from a single power-law
distribution of electrons (e.g. in a reverse shock or two-component jet), or a
thermal electron population, perhaps suggesting that one of the standard
assumptions of afterglow theory is violated. GRB 221009A will likely remain
detectable with radio telescopes for years to come, providing a valuable
opportunity to track the full lifecycle of a powerful relativistic jet.
