Somewhere, there exists a mansion where imaginary friends thought up by children become real, and once their children outgrow them, they return to the home to retire, with the hope that another child can befriend them. The home is run by the elderly Madame Foster, its lovable, kind founder; her rabbit imaginary friend Mr. Herriman, the strict rule-abider and business manager; and her granddaughter Frankie, who handles day-to-day operations.
When eight-year-old Mac is told by his own mother that he must stop believing in imaginary friends, he leaves his own friend Bloo at the house, but so as to stop him from being re-adopted, he agrees to stop by the house and play with him every day. The series has the story of Mac and his daily visits to the house and his experiences with the cast of eccentric characters, but all engaged in their standard "slices of life".