Tesseract

While the Tenth Doctor gives his new companions, Emily Winter and Matthew Finnegan, a tour of the TARDIS, the vessel is invaded by a number of dimension-hopping aliens. The Advocate places doubt into Matthew Finnegan's mind about the Doctor's morality, while the Tef'Aree have much the same effect upon Emily Winter. Meanwhile, the proximate cause of the invasion — the breech of the TARDIS by the Acari spaceship —forces the TARDIS interior to rearrange itself. Because the Doctor is psychically linked to the TARDIS, this trauma causes the Doctor to undergo temporary mental instability and his past selves begin to bleed through. Only by disengaging from the Acari ship and quickly restoring the proper console room will the Doctor's tenth persona fully reassert itself.