It's a regular underswap take, with some adjustments:
Frisk is in Chara's role, but Kris is in Frisk's role. Chara is gone.
While Napstablook is TECHNICALLY in MTT's role, Undyne fights you at the end of the CORTEX (CORE replacement) instead.
Mad dummy swaps with the Ruins dummy, Mad Dummy does not have a physical form. Mad Dummy is a red ghost called Angstergeist, And The Ruins Dummy simply calls himself "D".
Instead of fighting Mettablook in the Castle Ruins (Ruins replacement), you fight Angstergeist.
The annoying dog swaps with the bird that carries you over the gap.
Asriel and Flowey technically share the role of MK. Flowey is simply a flower-shaped monster that behaves the way UT Flowey pretends to. Asriel ran away from home after Asgore and Toriel's fight. These two fight you instead of a Greater Dog replacement to prepare you for your fight against sans. They do not do this you during genocide.
Instead of the amalgamates, you face a robot powered by DT in the true lab strangely resembling Spamton NEO. It was originally going to be a weapon to destroy the barrier, but the AI glitched and the experiment failed.
