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It is a remarkable fact that the shape and strength of the ionizing continuum of the most massive stars has NEVER been spectroscopically mapped comma at ANY redshift. Locally comma the sparsity of such stars and the Hydrogen opacity precludes observations comma and by the time redshift gives us access to these rest wavelengths in distant galaxies they are normally too faint comma even for HST. Here we propose to acquire comma for the first time comma spectroscopy of the ionizing continuum in a star-forming galaxy down to a restframe of 560A (or photon energies of 24 eV) by observing THE brightest known gravitationally lensed galaxy comma the Sunburst Arc. Our proposed observations - WFC3 grism spectroscopy - will directly probe the strength and spectral shape of the ionizing continuum in this galaxy. This will be the first-ever opportunity to directly compare the measured ionizing continuum against that inferred by the non-ionizing continuum and nebular emission lines. With this unprecedented observation comma we will directly observe the strength and shape of the ionizing continuum comma test whether the observed ionizing continuum is capable of producing the previously observed nebular emission lines comma inform the next generation of theoretical models of stellar populations comma and determine the impact of binary evolution on the production of ionizing photons. These observations will leave a lasting legacy for Hubble by mapping the unseen source of most extragalactic observationsdoublePoint the ionizing continuum of massive stars.