We present Hubble Space Telescope ( HST ) WFC3 imaging and grism spectroscopy observations of the Herschel -selected gravitationally-lensed starburst galaxy HATLASJ1429-0028 . The lensing system consists of an edge-on foreground disk galaxy at z = 0.218 with a nearly complete Einstein ring of the infrared luminous galaxy at z = 1.027 . The WFC3 spectroscopy with G102 and G141 grisms , covering the wavelength range of 0.8 to 1.7 \mu m , resulted in detections of H \alpha + [ NII ] , H \beta , [ SII ] , and [ OIII ] for the background galaxy from which we measure line fluxes and ratios . The Balmer line ratio H \alpha /H \beta of 7.5 \pm 4.4 , when corrected for [ NII ] , results in an extinction for the starburst galaxy of E ( B - V ) = 0.8 \pm 0.5 . The H \alpha based star-formation rate , when corrected for extinction , is 60 \pm 50 M _ { \sun } yr ^ { -1 } , lower than the instantaneous star-formation rate of 390 \pm 90 M _ { \sun } yr ^ { -1 } from the total IR luminosity . We also compare the nebular line ratios of HATLASJ1429-0028 with other star-forming and sub-mm bright galaxies . The nebular line ratios are consistent with an intrinsic ultra-luminous infrared galaxy with no evidence for excitation by an active galactic nucleus ( AGN ) . We estimate the metallicity , 12 + log ( O / H ) , of HATLASJ1429-0028 to be 8.49 \pm 0.16 . Such a low value is below the average relations for stellar mass vs. metallicity of galaxies at z \sim 1 for a galaxy with stellar mass of \sim 2 \times 10 ^ { 11 } M _ { \sun } . The combination of high stellar mass , lack of AGN indicators , low metallicity , and the high star-formation rate of HATLASJ1429-0028 suggest that this galaxy is currently undergoing a rapid formation .