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Plaskett's "star" appears to be one of a small number of short-period binarysystems known to contain a hot, massive, magnetic star. Building on the 2013discovery investigation, we combine an extensive spectropolarimetric (Stokes$V$) dataset with archival photometry and spectropolarimetry to establish theessential characteristics of the magnetic field and magnetosphere of therapidly rotating, broad-line component of the system. We apply Least-SquaresDeconvolution (LSD) to infer the longitudinal magnetic field from each Stokes$V$ spectrum. Using the timeseries of longitudinal field measurements, incombination with CoRoT photometry and equivalent width measurements ofmagnetospheric spectral lines, we infer the rotation period of the magneticstar to be equal to $1.21551^{+0.00028}_{-0.00034}$ d. Modeling the Stokes $V$LSD profiles with Zeeman Doppler Imaging, we produce the first {reliable}magnetic map of an O-type star. We find a magnetic field that is predominantlydipolar, but with an important quadrupolar component, and weak higher ordercomponents. The dipolar component has an obliquity near 90 deg and a polarstrength of about 850 G, while the average field strength over the entiresurface is 520 G. We update the calculations of the theoretical magnetosphericparameters, and in agreement with their predictions we identify clearvariability signatures of the H$\alpha$, H$\beta$, and He II $\lambda 4686$lines confirming the presence of a dense centrifugal magnetosphere surroundingthe star. Finally, we report a lack of detection of radial velocity (RV)variations of the observed Stokes $V$ profiles, suggesting that historicalreports of the large RV variations of the broad-line star's spectral lines maybe spurious. This discovery may motivate a fundamental revision of thehistorical model of the Plaskett's star as a near-equal mass O+O binary system.

The magnetic field and magnetosphere of Plaskett's star: A fundamental shift in our understanding of the system