Solar flares are energetic events in the solar atmosphere that release an enormous amount of energy in a short duration. This movie shows an interesting example among the best flare observations that IRIS has captured. This M3.7 class flare occurred on 2017 September 9 in the super Active Region (AR) 12673, which produced the largest flares of this solar cycle (Cycle #24), including the 2017 September-6th X9.3 and September-10th X8.2 flares. The movie shows the AR evolution over a long, six-hour duration, in which a number of flares of various sizes occurred. The major M3.7 flare occurred toward the end of this movie at about 10:50 UT, starting with some brightening and an erupting loop system in the upper (northern) portion of the image, situated between two sunspots. (At this time, the AR was very close to the southwest limb of the Sun, and thus the sunspots appear elongated in the north-south direction due to fore-shortening.) A bright flare ribbon rapidly grew and extended toward the south, right under the IRIS slit that captured some interesting spectroscopic data with rarely-used full CCD readout. An arcade of post-flare loops can be seen later, e.g., at 11:25 UT. Detailed studies of IRIS observations like this will shed light on a wide range of physically important but poorly understood processes involved in solar flares, such as plasma heating and particle acceleration.