Frisk was on a genocide route (on hard mode), and nearly finished it. They had to kill Sans, so they could continue with the route.
Chara started gaining control a little before Frisk was about to press the RESET button, Chara then gained full control of Frisk and fought Sans.
Sans fused with Frisk, giving him an incredible power boost. After that, they proceeded to defeat Chara.
While Frisk was desperately trying to kill Sans. But then, Frisk wanted to RESET after losing to Sans several times. However, Chara came and decided to take control over the situation. It was proven that Chara was more powerful than Frisk (possibly because they were filled with HATE). Chara overpowered Sans and easily dodged all of his attacks. Sans soon, started to become even more tired during the battle. Chara was about to kill Sans by slashing their knife at him. And if it wasn't for Frisk who decided to stop Chara by blocking their fatal blow, Sans would have been killed. After that, Frisk gave their soul to Sans in order to defeat Chara. After defeating Chara, Sans resets the timeline after calling Chara a "Megalomaniac".
|Pre-Glitchtale Battles||Chara vs Asriel ◆ The Great Human and Monster Wars ◆ Agate Lightvale vs Copper Lightvale ◆ Integrity vs Asgore ◆ Flowey vs Sans ◆|
|Season One Battles||Frisk & Chara vs Sans ◆ Gaster vs Frisk & Sans ◆ Chara vs Sans & Flowey ◆ Chara vs Omega Flowey, Sans, Frisk & Gaster ◆ Asriel vs Chara◆|
|Season Two Battles||The AMD's trap ◆ Battle on The City ◆ Gaster vs Bete Noire ◆ Undyne vs Bete Noire ◆ Frisk vs Bete Noire ◆ Asriel & Bete Noire vs Frisk, Toriel & Asgore ◆ Mettaton vs Bete Noire ◆|