"You did the right thing, Frisk". - Chara, at the end of the True Pacifist Run.
The True Pacifist route is where Frisk reforms all the monsters and gets their happy ending.
How it all goes down
The True Pacifist route is when Frisk disobeys Flowey's orders to stop the monsters, and become friends instead. It starts off when Frisk shows Mercy to the monsters of the Ravaged Mines. After making it through the Ravaged Mines without "doing a violence", you make it to Toriel's house. She then finds you trying to escape and attempts to kill you, but then fails, and attempts to spare you.
You leave the Ravaged Mines and make it to Tundra Forest. You then fight Sans, who, during battle, tells you about LV and other stats. Sans did not kill you due to Papyrus' order, while Papyrus then sneaks up on you and sees you. He then challenges you to solve his puzzles along the way. The dogs fight you as you progress through Tundra Forest. When you reach Papyrus, Sans seems to have disappeard. He talks about being captain of the royal guard and wanting you dead. You then spare him and teach him kindness, and he begins to learn it pretty well.
When you reach the Waterfall, you will encounter more monsters (including Monster Kid) and spare them. In Wasteland, Mad Dummy will still be encountered as normal. When you encounter Undyne, she complains about being kicked out of the royal guard and seeking revenge. In the fight, she is in her normal form. You cannot spare her so you have to run away from her and pour some water onto her when she falls on the bridge to revive her from the heat.
You then encounter Alphys, who orders Mettaton to kill you as a beta test for human eradication. After you make it to the Mettaton fight, as part of the "Lessons of violence" show, he tries to kill you like he was told to. He first starts with puzzles and quizzes, and then finally changes into his Mettaton EX form, but ends up failing. Alphys, looking back at what the player has changed this world into, decides to change her mind and shows the player her True Lab.
Using an elevator in the lab, you then pass through the judgement hall and find Sans. Starting from this point, FIGHTing would only result "But your DETERMINATION made you MISS." After battling Sans, the player encounters Asgore. Being disgusted by the player's actions, he destroys all the buttons except for FIGHT, then accidentally destroys it as well. Flowey then restores it, but as the player refuses, Flowey restores the ACT button instead. Everything is then restored to normal. Asgore then shows the player his black and red castle as their new home if they chooses to stay. If the player chooses to leave, all the important monsters make a call for them, and tells them what happens after then.
Chara, being reincarnated, appears, telling the player that they did the right thing. Chara also tells the player that sometimes, they would want to "go back to the beginning", and offers them a "True Reset" (and a "Continue" if the player wants to explore this place first).