Roots: The Three Doctors and The Five Doctors.
Goofs: The subtitles on the Series 3 DVD spell zeiton crystals as Ziton crystals.
How do a black hole and a supernova cancel each other out?
If the Doctor remembers this incident, and he's told himself that the Master just turned up again, then why did he think that no other Time Lords survived the Time War throughout the previous three series?
Technobabble: “The two of us together has shorted out the time differential” is an explanation of why the fifth Doctor looks so old.
“Venting the thermobuffer, drawing the Helmic regulator, and just to finish off, let's fry those Zeiton crystals.”
Dialogue Triumphs: Tenth Doctor: “Fair play to you. Not a lot of people can carry off a decorative vegetable.”
Fifth Doctor: “That's a paradox that could blow a hole in the space time continuum the size of... Well, actually, the exact size of Belgium.”
Fifth Doctor: “You're. Oh, no.”
Tenth Doctor: “Here it comes. Yeah, I am.”
Fifth Doctor: “A fan.”
And, well, practically the whole script.
Continuity: Both the fifth and tenth Doctors wear glasses to make him look brainy. The New Series TARDIS redesign is due to changing the desktop theme. The fifth Doctor thinks this one is Coral, and is worse than the Leopard Skin.
The crash with the Titanic is due to the Doctor forgetting to put the TARDIS shields back up after he has repaired it from being a Paradox Machine.
Links: The fifth Doctor mentions L.I.N.D.A. (Love & Monsters). The tenth Doctor mentions rebuilding the TARDIS (from the Paradox machine of The Sound of Drums/Last of the Time Lords), Nyssa, Tegan, and the fifth Doctor's encounters with Cybermen (Earthshock, Spare Parts), Mara (Kinda, Snakedance), Time Lords (Arc of Infinity, The Five Doctors), and the Master (Castrovalva, Time-flight, The King's Demons, Planet of Fire). We hear the Cloister Bell.
Location: The TARDIS.
The Bottom Line: ”Yes you are. You are the Doctor.” This scene is absolutely marvellous, with some top class banter between the two Doctors. However, my suspension of disbelief is seriously stretched when Tennant starts telling Davison “you were my Doctor”.