wiki:
"As of the New series, the Doctor has encountered the Face of Boe (as originally presented) three times. In "The End of the World" (2005), set five billion years in the future, the Face of Boe sponsored the safe viewing of Earth's destruction by the expansion of the Sun. The event was sabotaged, but the Face of Boe survived along with most of the guests, thanks to the Ninth Doctor's intervention.
In "New Earth" (2006), the Face of Boe is apparently dying of old age in a New New York hospital in the year five billion and twenty-three. His nurse, one of the Sisters of Plenitude, Novice Hame, tells the Tenth Doctor that the sleeping Face sometimes sings "ancient songs", which she hears telepathically, and relates the legend that just before dying the Face will reveal his greatest secret to "one like himself", "A wanderer without a home" (referring to the Doctor). When the Doctor later asks the Face of Boe about the legend, the Face of Boe claims that the "great secret" can wait for their third and final meeting, and enigmatically teleports away to an unspecified destination."
