Synopsis: Rose wants to see the Mona Lisa. The Doctor obliges--but doesn't exactly take her to the Louvre. Instead, they end up in the 37th century, where they find the classic piece moved to a trans-dimensional art museum.
There, they encounter a charismatic non-human artist named Cazkeif the Transcendent, who invites them to use the museum's headsets to learn more about the works of art on display.
The Doctor declines as he probably knows more about the art than the museum itself, but Rose's inquisitive nature gets the better of her. Within seconds of slipping on the headset Rose has disappeared--along with most of the humans in the building.
Investigating, the Doctor discovers that Cazkeif has wired the headsets into his ship's distress beacon, hoping to increase the amplification of his signal with every mind "listening" to the headphones.
The Doctor distrupts the effect with his sonic screwdriver, returning all the humans to their rightful place inside the machine--but not before coming under the "influence" of Cazkeif himself. Though he initially thought the alien was up to no good, it now turns out that he is a true victim of circumstance, having crash-landed on Earth with no hope of escape other than trying to make his rescue beacon send a signal with the assistance of the few unwitting humans who decide to put on the headphones. Since the Doctor is himself alone in the universe, and Cazkeif's methods were basically harmless, he is sympathetic. He gives the alien a lift back to his home planet in the TARDIS. Sadly, in the time that Cazkeif has been on Earth, his planet has been destroyed.
With nowhere else to go, Cazkeif and the Doctor return to the museum on Earth. Cazkeif fully expects to endure the wrath of his unwitting human "assistants", but instead finds them enthusiastically applauding what they are now perceiving as a masterful work of performance art. Cazkeif remains on Earth, basking in this newfound acclaim, and hoping to memorialize his species with new art.