:MKV is a container format that often holds multiple subtitle tracks. You can use tools like MKVToolNix or gMKVExtractGUI to "demux" (pull out) the subtitle files (usually .srt or .ass format) from the video without needing to re-encode it.
Be extremely cautious when searching for specifically named video files on the web. A common tactic used by malicious sites is to disguise as an .mkv file.
If your goal with "deep text" is to extract the dialogue or subtitles from the video, here are the standard methods: