The Interface Region Imaging Spectrometer (IRIS) is the first solar instrument to observe $sim 10$ MK plasma at subarcsecond spatial resolution through imaging spectroscopy of the Fe XXI $lambda$1354.1 forbidden line. IRIS observations of the X1 class flare that occurred on 2014 March 29 at 17:48 UT reveal Fe XXI emission from both the flare ribbons and the post-flare loop arcade. Fe XXI appears at all of the chromospheric ribbon sites, although typically with a delay of one raster (75 seconds) and sometimes offset by up to 1$^{primeprime}$. 100--200 km s$^{-1}$ blue-shifts are found at the brightest ribbons, suggesting hot plasma upflow into the corona. The Fe XXI ribbon emission is compact with a spatial extent of $< 2^{primeprime}$, and can extend beyond the chromospheric ribbon locations. Examples are found of both decreasing and increasing blue-shift in the direction away from the ribbon locations, and blue-shifts were present for at least 6 minutes after the flare peak. The post-flare loop arcade, seen in Atmospheric Imaging Assembly (AIA) 131 AA filtergram images that are dominated by Fe XXI, exhibited bright loop-tops with an asymmetric intensity distribution. The sizes of the loop-tops are resolved by IRIS at $ge 1^{primeprime}$, and line widths in the loop-tops are not broader than in the loop-legs suggesting the loop-tops are not sites of enhanced turbulence. Line-of-sight speeds in the loop arcade are typically $<10$ km s$^{-1}$, and mean non-thermal motions fall from 43 km s$^{-1}$ at the flare peak to 26 km s$^{-1}$ six minutes later. If the average velocity in the loop arcade is assumed to be at rest, then it implies a new reference wavelength for the Fe XXI line of $1354.106pm 0.023$ AA.
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