The story... well, the first video is a perfect hook for the premise, so just watch that. (Don't be dissuaded by the fact that it's shot "found footage" style; it mostly avoids the annoying trappings.)
If you didn't have time to watch a two-and-a-half minute video, but you're a quick reader,
Spoiler: show
It's a damn compelling hook, but I've seen great premises squandered before on amateurish web shows. Luckily, Hi I'm Mary Mary earns gold stars in every respect. The woman playing Mary either has acting experience or is just a total natural on camera, because she's believably charming and likable, and none of her movement or speech comes off as unnatural. Together with how she frames her isolation, the rules she establishes about how the strange house works, and the personal details that slowly come to light, this lends the series' world an air of unique believability that makes it very easy to get immersed and feel like you're trapped in there along with her.
Which is great, because the atmosphere and the stylish way it's presented (the creator clearly also has some experience with cinematography, editing, and special effects work) make the non-psychological horror elements all the more intense. When night falls and the house's inhabitants come out to play, Mary is stuck in a variety of suspenseful scenarios that must have been hellish to set up on an indie web series budget. The effects are a little wonky every now and again, but the seamless manner in which Mary reacts to them and the knowledge of how much work had to have gone into them make them pop despite this (an early scare involving a mirror should be unbelievably corny, and it totally is when it first happens, but the execution, reaction, lasting effect, and subtextual meaning turn it into a great moment). I especially love everything that happens in "DO NOT TOUCH", currently the longest video and one of the most creative.
It's great. It's great. Go watch it. I'm always in the mood to support upstart creators, and this woman is going places.