At almost the halfway point of the 2008 season, the Doctor, Donna, and Martha have been kidnapped by the TARDIS and hauled off to a planet called Messaline.  Perhaps the TARDIS itself has grown tired of having 90% of the stories set on Earth.  There's even a bonus alien race!

...Which is trying to kill off all the humans.

Moments after landing, the Doctor and Donna have witnessed a gunfight, Martha has been kidnapped, and the Doctor's hand has been forcibly shoved into a cloning device that whines, shines, and spits out Georgia Moffat, a fully grown soldier already indoctrinated with the Messaline mythology, fighting techniques, and the need to kill as many Hath as she can get in her gunsights.  Donna is delighted, telling the Doctor he's no longer alone, as he has a daughter.  In one of the few departures from fanfiction, the Doctor isn't thrilled - he snaps back that every time he looks at the girl, he thinks of his family "and the hole they left and the pain that filled it."  He dismisses the soldier as a "generated anomaly" and focuses on trying to get Martha back.  Donna, unbowed, promptly turns "generated" into the name "Jenny," which the soldier adopts with a grin.

She does most things with a grin.  Georgia Moffat is as perky and adorable as a basket full of puppies, and she is also Peter Davison's child, meaning that the Doctor's daughter... is played by the Doctor's daughter.  

In many ways, that meta is the most pleasing bit.  Delivering another uneven episode (after 2007's The Lazarus Experiment), Stephen Greenhorn has written some memorable moments, but the story itself falls apart at any attempt at examination.  The humans (whose HQ looks suspiciously like the theater used in Torchwood's "Captain Jack" episode) want to wipe out the Hath and get control of something called "The Source." The Hath, gilled creatures with bubbling tanks creepily bolted to their faces, want to do the same in return.

As for our heroes, the Doctor follows a predictable pattern of resenting Jenny and trying to teach her out of her knee-jerk violence while he slowly becomes fond of her despite himself.  (It's like watching the UNIT years in fast-forward.)  Donna follows an equally cliched pattern of being mouthy and ignored until her temp working experience allows her to pull a key plot point out of the air.  And Martha, who should have considered a short jaunt across the blasted face of the planet a stroll in the park after walking a decimated Earth, stumbles and flinches her own path to the Source accompanied with a Hath that only she can understand, the TARDIS translation inexplicably not being extended to the audience for the first time ever.

And then there's Jenny.  Even the people who liked her - I'm one of them - can't deny that she's the Mary-est Sue who ever Sued, especially as Greenhorn wrote her not one, but two blatantly emotion-jerking endings.  

He also wound up the plot with a blatant ripoff of Star Trek II, The Wrath of Khan, a bad move when half the fandom is old enough to remember watching the original in movie theaters.

If you can hang your disbelief high enough, The Doctor's Daughter is a lot of fun in that "behold the power of cheese" in which Doctor Who excels.  Everyone acts their heart out, but there's only so much they can do with what they've been given.