The first half of a stunning two-parter.  I love how writer Paul Cornell just throws you into things and you get to piece together what happened to the Doctor as the story unfolds.  Not only is this an incredibly important story for the Doctor's character, but it also establishes major Time Lord lore in the form of the fob-watches.  It's also quite possibly Martha at her peak.  She very much carries this episode and the one that will follow for the vast majority of the run-time.  Martha is so hopelessly put in this bad position - she is a woman of colour at a time where that makes her powerless and voiceless.  Even John Smith - the Doctor now unable to recognize or remember her - clearly views Martha as a subordinate.

So she's left to pick up the pieces with no plan, no easy solution, and no full awareness of who she can trust or what she can do.  It's a nightmare few other companions have endured.  Human Nature is solid as hell overall.  It does a great job of putting us in this time period and it is full of great character work even beyond Martha and the Doctor.  I really like the romance between Jenny and "John."  In a show full of one-story romances, this is easily one of the most stand-out and significant among them.  Even the boy Latimer was done pretty well, though his role as the super-special kid destined to help the Doctor remember (while navigating bullies) is a little tropey here.  I like the angle these are all future World War 1 veterans that's explored more in the next episode, so it all works out anyways.

Plus the Family of Blood make for a very sinister set of villains.  They too are memorable even though they have a lot of competition in this franchise as one-off monsters.  And it all really, really helps that the next episode, The Family of Blood, absolutely nails paying off everything set up here.