Ark, the great -nibling of Era, who fell down the hole in Mt. Ebott 50 years ago, follows in their grankle’s footsteps (quite literally) and takes a tumble of their own. There, they meet Undyne, a former soldier who has retired to a quiet life in the ruins, under the orders to bring any fallen humans to the capitol. But over the week or so that Ark stays with her, she can’t bring herself to send them off to have their soul reaped, and instead hands them a small spear and offers a small protection charm. With those in hand, Ark is told to find a way home, by any means necessary.
They travel to Snowdin, meeting a pair of ghosts along the snowy paths - an excited pink one named Mettablook (though he insists on just being called Mett), and a chill blue one named Napstablook. Mett is a human fanatic, and continuously tries to get Ark to tell him about human culture while they travel towards town. If he’s not talking about the stuff he finds at the dump, he’s going on and on about his favorite tv personality. Napstablook manages to tell Ark that they make lofi hip hop remixes in their spare time.
Once they get to town, they bump into Asgore, a friendly, kind shopkeeper, with a gaggle of kids behind him. The shopkeep and human head towards Muffet’s Bar and Grill, and Asgore tells Ark about monster history, and how they’ll have to have a monster’s soul by the end of their journey if they want to get past the barrier. Ark is adamant they’ll be fine, and they’ll find another way through.
After leaving Snowdin with Mett in tow, Ark gets their first encounter with Grillby in Waterfall - Grillby’s a hardened soldier with a bullheadedness and a DETERMINATION to stop the human. During a chase, a young goat child named Asriel distracts Grillby long enough for Ark and Mett to hide out on a snail farm, where they meet the owner, Toriel. She takes in the two for the night, and they have a big dinner with Tori, her son, and her wife, Muffet.
Moving through Waterfall, the duo narrowly misses more attacks from Grillby, and only manage to escape when his enchanted armor’s power starts to dwindle from having no cool down time, causing him to become more vulnerable to the moisture in the air. Ark and Mett, in an act of compassion, drag him through to Hotland where he can dry back out and let his armor’s magic recharge.
Now in Hotland, the pair enter a science lab, and get immediately accosted by a tall, boisterous....skeleton ? by the name of Gaster. They’re not all there, and they’re a little drippy, but they’ve heard all kinds of things about the human and they’re begging for the chance to study a human SOUL. Before they get the chance to start inspecting, a shorter skeleton named Sans pushes them out of the way and insists that the human meets his brother Papyrus and do a segment on his talk show to clear up some rumors.
Ark and Mett are confused, but don’t have much time to ask questions as Papyrus himself appears and drags them to another room that’s set up with a desk and some armchairs. Once cameras start to roll, it’s revealed to Ark that many of the monsters they’ve met have been mistaking them for a human from 20 years ago - one named Era. All kinds of crazy theories that Ark was Era’s spirit come to avenge the monsters or whatever have been all over the monster internet, and Papyrus wants them to be able to clear things up.
A picture of Era flashes on a screen behind them, and Ark recognizes them - it’s their great-Ankle who went missing when their grampy was still young. The monsters at home are shocked ! What are the odds 2 humans from the same family would fall down here ? It’s then that the story of the dearly missed Prince Monster Kid and the fallen human Era are told. Much like in canon, Era was taken in by the royal family, this time consisting of Alphys, her cousin Monster Kid, and her right hand guard Undyne. After Era fell gravely ill with a sickness their body couldn’t fight, Monster Kid and Era fused their souls and emerged into the above ground in order to look for a cure, or at least take Era back to their home town to see their loved ones one last time.
Unfortunately, the humans there thought Monster Kid had attacked Era and hurt them, and began to attack Monster Kid in turn. Injured fatally, Monster Kid picked up their siblings body and returned home, barely making it back to Alphys before they turned to dust. Alphys, with both the kids she loved now dead, woefully vowed to avenge their deaths and free monsters from the underground and the plight of humanity. She sent Undyne to the ruins to watch and wait, and so over the decades they have reaped 5 human SOULS.
Determined to make amends to Alphys, the whole of monster kind, and their own family’s lost history, Ark heads towards the castle with Mett, and now Papyrus. Along the way, the other monsters they meet offer help, charms, spells, and anything they can give, heartbroken to know that neither of Era’s families had gotten closure until now about their disappearance.
At the castle, the trio encounters Alphys, sitting and weeping in a flower bed of golden petals. She spoke lowly to someone they couldn’t see, and when she turned to greet them, there was nobody there. She wiped her eyes and gently hugged Ark, voice hoarse as she apologized, but she had a duty to her people.
They enter the next room, and it’s revealed... that Alphys won’t be the one being fought. It’ll, instead, be a teched out Flowey, the lost soul of Monster Kid, who’s powered by the other human souls. Once they claim Ark’s, they plan to blast the barrier to bits and lead the charge against the humans that caused so much damage.
During the fight, Ark keeps getting flashes of the past from the dead human’s memories - a ballet dance here, a cooking lesson there - and finally, memory upon memory that could only belong to the family member they never knew. They appealed to the child inside Flowey, the one who just missed their sibling and wished things could have changed. Ark’s soul glows bright, resonating with the ones surrounding Flowey... and when the flash clears, Monster Kid is standing there, instead. They’re sniffling, ashamed and depressed, and with the last of their power they decimate the barrier.
It won’t be long until they become a flower again, and they’re enveloped in a hug by Ark, Alphys and Papyrus joining not long after. As the Monster Kid leaves, other monsters swarm the entrance to the room, eager to step out into sunlight for the first time in centuries.
Alphys stays behind, hoping to help Monster Kid, and a few others stay to assist, but mostly monsters run out with glee. The main group linger, not wanting to leave their newfound friends behind until they were all ready to go. Ark heads in the direction Monster Kid went, and when they catch up, they promise that Monster Kid will never be alone and sad ever again. The human stays until Monster Kid becomes Flowey once again, before gently digging them up and carrying them all the way back to the castle, and soon, out to the above ground.
