The pre-title sequence to this episode quickly explains the title's REAL meaning: that "The Doctor's Daughter" is not a family member returning from the ashes of Gallifrey, but an anomaly generated by extracting a cell from The Doctor's hand and then growing a clone from it.
Ironically, The Doctor's daughter is portrayed by Georgia Moffett, who is Peter Davison's real life daughter. So, The Doctor's daughter IS The Doctor's daughter.
This episode scored an 8.5 because it was full of classic Who flavor from start to finish. Another world, underground tunnels, a war between the humans and the fish-like Hath, a slate of mysteries to solve, one of The Doctor's companions separated from the group, Donna once again shows off how clever she is, and The Doctor using a little wind-up mouse to distract a guard were all generous amounts of key ingredients that kept me on the edge of my seat....well, actually I watched from the foot of my bed, but you get the idea.
After seeing this one for the first time, I recall instantly considering it one of my favorites, and it still is. The Fourth Series seemed to be the best of the Tennant years as far as being parallel to what Doctor Who is all about.
The only dispute I had was at the very end, where Jenny is shot and The Doctor gets a bit "Captain Kirk preachy." I did wonder if Jenny's revival was a rip off from "Star Trek: The Search for Spock," where a similar terra-forming device, GENESIS, was used and inadvertently reanimated a dead Spock.
It was Moffat's decision to keep Jenny alive, and hats off to him for that. At least we know there is one other Time Lord out there somewhere.
Brilliant episode, and definitely the best of the season so far.