ffmpeg -i "The.Flash.S01E08.mkv" -c copy -map 0 "The.Flash.S01E08.mp4" Keeps good quality at ~½ the size of original H.264.

ffmpeg -i "The.Flash.S01E08.mkv" \ -c:v libx265 -crf 24 -preset medium \ -c:a aac -b:a 128k \ -c:s copy \ "The.Flash.S01E08.hevc.mp4" -crf 24 = good balance. Lower = better quality (18-28 range). From 00:38:00 for 90 seconds:

I’ve broken this into — choose the one that fits your goal. 1. Remux MKV to MP4 (no re-encode, fast) If you have an MKV file (common for this episode) and want an MP4 for better device compatibility: