The Longest JourneyEdit
A door near the Fringe Cafe in Venice, Newport. April says that, though she works in the Fringe and spends a lot of time hanging out there, she's never seen anyone go in or out of it. She follows this with: "And if I was Nancy Drew, I might even care."
When she is pursued by the Vanguard, April runs to the Fringe, only to find herself cornered. When the Vanguard soldiers close in on her, the Mystery Door flies open, revealing a portal that resembles a Shift. April runs through it and finds herself in the House of All Worlds, where she is greeted by Lady Alvane.
In Dreamfall, Zoe Castillo simply comments: "What a mysterious - but completely irrelevant - door." The Mystery Door does not appear to have any inherent power, only that which is lent to it by Lady Alvane.