Our Final Heartbeat is a Doki Doki Literature Club mod with the premise of going back through DDLC and averting everyone's deaths.
I feel bad saying I don't recommend this mod, because I puke at the thought of some of the mods that I gave ambiguous verdicts to being better than this one. The reason for my negative verdict here is that it's kind of a strictly worse Doki Doki: Salvation.
It copy-pastes almost all of Acts 1 and 2, it doesn't handle the resolutions to the characters' problems particularly well, and the plot is an incomprehensible mess. I'm not sure if there's a possible headcanon that makes it work:
In the beginning, "Monika" tells you that you're going to go back and save everyone. You restart the game and Act 1 plays out exactly the same up until the last conversation with Sayori. The way you save her isn't by anything particularly key that MC says but by Monika having a change of heart and ceasing to amplify her depression. Monika thanks you in an OK-box for saving Sayori and the game moves to Act 2. Act 2 plays out the same until the very moment Yuri is about to stab herself, where you save her in the moment and then she implies awareness of the 4th wall and the screen immediately fades black to "Monika" thanking you again. Then a brand new section of what might have been Act 3 where only Natsuki and Monika are around plays out and this time, you don't even make a choice despite being explicitly told that you'll have to; Monika just saves the day all for you (and this time around there's no sign that she was behind what happened to Natsuki in the first place). Finally you get a brief and unsatisfying ending with everyone that lasts about 10 lines where nobody talks about what just happened. It's revealed that the "Monika" that's been guiding you is not the original Monika, but "Monik", an AI created by the mod author, and they're both there. (But at the same time Monik is Monika. I don't think the writer understood that that's a binary question.)
This was one of the first mods I played. It was back when I had recently finished the original and I was so desperate to see the girls saved that I probably gave it a lot more credit than it deserved. So that shaped my initial review (not the one you're reading now). Also, judging by what I found on rereading the scripts as of this writing, my memory had become warped significantly in the mod's favor over the time between playing it and writing said review. (I'm worrying now that the same thing happened to Salvation... but Salvation's too long to replay or read to find out, and it was a lot more recent, so hopefully not.)
I should also mention that the mod won't work without having a save file with the original game's golden ending achieved (which doesn't really make sense as the mod is about fixing what happened in the original game's normal ending). The developers offer a download of the save file on their discord, but if you don't want to join for that it should be easy enough to get around this by using the developer console to set persistent.clearall to True (though you'll probably have to decompile and edit the scripts to do this - see my main DDLC page for an explanation of how to do that).
My biggest point of praise is MC's interactions with Natsuki in phase 3. Their relationship reflects Natsuki's original personality accurately and is full of intense banter but still wholesome. I like how they made the weekend scene less objectionable. It still happens, but Natsuki continues to fight back psychologically even while physically defeated, and eventually does break free.
There are two new files to see. mc.chr is a PNG of a flower in greyscale, and monik.chr is the binary-encoded ASCII text "I'm still here for you. Let's save them, okay?"