based on MLP's episode, A health of information.
Sans and Undyne were at the Lightbright lake. Undyne was training swimming while Sans was only watching.
"The day is so fresh today!!!" Sans said.
"Yes, I need those Oohlala dandelions for my tea." Undyne said.
Suddenly, Undyne fell on the water.
"Are you okay?!" Sans said, pulling Undyne out of the water.
"Yes, I'm ok." Undyne said.
A red flower that looked a narcissus, threw a pink smoke on the air.
Sans and Undyne coughed.
"I feel so nauseous..." Undyne said, falling on top of Sans.
Undyne was at the hospital.
"The symptoms are from a uncommon disease called 'Mortallis Flu'." The doctor said.
"Can you explain more about it?" Sans said.
"The person will have spots that resembles lightnings marks when they hit a person, but with a black color. If Undyne don't get cured, she will have a heart attack, because when days passes by with this disease, the hearts starts to beat more faster, like if the person was running." The doctor said.
"And this is all my fault..." Sans said.
"Sans, I do not blame for this, accidents happens all the time.
"For the cure, you'll have to find Sugardaisies, Potatomato seeds, leaves from a Pinegarland tree, and others you can see on this giant list." The doctor said, giving a list to Sans.
"If this is to cure Undyne, I'll do it." Sans said.
Sans was at Alphys' house.
"Hi, what are you two doing?" Sans said.
"Baking a pistachio cake!" Papyrus said while putting a egg on the cake mix full of other things.
"I need help to find those ingredients here, guys." Sans said, giving the list to Alphys.
When Alphys finished reading the list she looked at Sans with a saddened look.
"Those ingredients are hard to found here on Hotland, so prepare yourself." Alphys said, grabbing a bag and bird masks.
Everyone was on a swamp. Sans was trying to catch some Sugardaisies.
"I must have focus to catch those freakin' flowers!!!" Sans said, struggling to catch one of the pink flowers.
Sans finally picked the flowers by using his telekinesis.
"Next ingredient is Potatomato seeds." Alphys said.
Sans was picking some seeds from a giant green fruit that had pink leaves, like a carrot.
"I did it!!!" Sans shouted.
"Next ingredient is leaves from a Pinegarland tree." Papyrus said.
Sans was on the top of a green tree with brown and red leaves, catching the leaves.
"That was difficult..." Sans said, panting.
Sans was reading the list, which it said the last item was a seed of a green Sapphirsuki flower.
"I'll go after it, to save Undyne!!!" Sans said.
Everyone was waiting for Sans, when Alphys appeared, on the door, with Sans on her arms, full of black marks on his whole body.
"W-what happened with him?" A maned wolf girl called Marina said.
"I saw him next to a giant green flower, fallen on the floor." Alphys said.
"I...have...to...save...Undyne..." Sans mumbled.
"You are not ok!!! You caught the Mortallis Flu!!!" Marina shouted.
"But I have to save Undyne..." Sans said.
"Maybe she is taking care of herself." Marina said.
"I'm not accepting this as a reply, Mrs. Marina!!! I'll save Undyne!!!" Sans said, running.
Sans mixed the ingredients and put some caramels and chocolate on it, to finish it, and honey.
"They say that sugar helps getting rid of that flu." Sans said.
After drinking the remedy, and getting better, Sans quickly ran to Waterfall.
Undyne drink the whole remedy, and the spots disappeared.
"I feel so good. Thanks Sans, I am very grateful for what you did for me." Undyne said.