Shigeo Kageyama (nicknamed "Mob" which is the Japanese term for background character) is a young boy who has extremely strong psychic powers (called "espers" in this series). Mob doesn't use them very often because he can't fully control them and he is anxious about hurting people. His powers manifest themselves when he is under duress so he tries to live an emotionless existence. Mob becomes the student of a charismatic fake psychic called Reigen who mentors him while simultaneously using his powers to bolster his Talk About Spirits Agency. Reigen isn't a bad guy, in fact, he cares about Mob quite a bit.
All of the issues that the characters face in Mob Psycho 100 stem from the fact that they are all suppressing emotions or desires. Mob's younger brother Ritsu is secretly jealous (and frightened) of his brother psychic abilities. A rival esper named Teru is deep down afraid of being a "commoner" and flaunts his powers whenever he can to maintain his popularity. During each episode Mob has a Psycho Meter that is shown with a numeric percentage. When outside influences pressure him the gauge rises and when it gets to 100% he goes into berserker mode where he is basically a God. It isn't always just anger either, it can be fear or even sadness. He is trapped in a self-made prison where he perpetuates a vicious cycle of pressure and release with no way of controlling it or the outcome.
Story aside, the look and animation quality of the series is stellar. The animation was handled by Bones and the psychic fights are amazing and dazzling to watch with lots of psychedelic colors and imagery. When Mob goes into full psycho mode he is shown as nothing but charcoal grey pencil scribbles as if he is drawing power from the animation itself. There are also tons of visual gags and jokes--only the last few episodes get extra serious in tone. The music by Kenji Kawai is excellent and the opening theme 99 by Mob Choir is catchy as hell. It's a twelve episode arc so there are no filler episodes and the pacing is perfect. It does take a bit to get going but once it does it ramps up nicely.
The theme of "breaking out of your shell" is a common one in Japanese anime, and with the societal pressures being what they are in that culture, it comes as no surprise. Mob goes through a interesting character arc where he discovers that it's okay to feel upset or scared because you will always have people willing to help you out. Though he has unlimited power, his true power comes from the support of his friends and family.