We present wide-field mapping at 850 μm and 450 μm of the z = 2.85 protocluster in the HS 1549+19 field using the Submillimetre Common-User Bolometer Array 2. Spectroscopic follow-up of 18 bright sources selected at 850 μm, using the Northern Extended Millimeter Array and Atacama Large Millimeter/submillimeter Array, confirms that the majority lie near z ≃ 2.85 and are likely members of the structure. Interpreting the spectroscopic redshifts as distance measurements, we find that the submillimeter galaxies (SMGs) span 123 pMpc2 in the plane of the sky and demarcate a 3600 pMpc3 "pancake"-shaped structure in three dimensions. We find that the high star formation rates of these SMGs result in a total star formation rate of 20,000 M⊙ yr−1 from just the brightest galaxies in the protocluster. These rapidly star-forming SMGs can be interpreted as massive galaxies growing rapidly at large clustercentric distances before collapsing into a virialized structure. We find that the SMGs trace the Lyα surface density profile. Comparison with simulations suggests that HS 1549+19 could be building a structure comparable to the most massive clusters in the present-day Universe.