Here's a plot synopsis taken from the games official backer page.
The original game follows the story of a young psychic named Razputin in his quest to join an elite group of international psychic secret agents, the Psychonauts. He runs away from the circus and breaks into their secret training facility, Whispering Rock Psychic Summer Camp. As he begins his training by psychically delving inside the consciousnesses of his tutors and those around him, he realizes all is not as it seems, and soon embarks upon a psychic odyssey through a variety of levels set inside the minds of misfits, monsters, and madmen.
But the story of Psychonauts is only the start of Razputin’s adventures. The game ends on a cliffhanger with the promise of more to come… Now it’s time to tell that story.
In Psychonauts 2, Raz realizes his dream and visits Psychonauts Headquarters. However, when he gets there, he finds it's not the perfect place he expected and quickly realizes that the Psychonauts need him more than he needs them.
Psychonauts 2 will feature a new hub world inside Psychonauts HQ. You’ll access new mental worlds as Raz peeks inside the minds of a host of new characters who need his help to combat their inner demons and unravel their deep-seated emotional issues. Raz will hone his secret agent PSI-abilities—and learn new ones too—using them to solve mysteries and uncover evil plots.