About 11 years ago, the great Warrior, Gregor the Overlander, roamed the streets of Regalia on endless quests. After very little activity, he never returned, and the Underland was left curious to what the famous adventurer had been doing. They sent letters, but, of course, Gregor would have never gotten them, since he had moved to Virginia a while ago.
In his old apartment, a new family moves in and discovers the loose laundry gate, calling and engineer in to fix it for them. The gate would never be fixed, though, as the curious engineer discovers a pile of letter left just behind the gate and falls in after not being cautious enough.