The Runaway Bride
Our introduction to the tornado that is Donna Noble. She starts off very angry - seemingly at the Doctor, but she's also angry at the way her life has been going: dead-end job, unsatisfying love life until her recent engagement, and her fiancé turns out to be an alien who is just using her. On top of that, she finds herself in the TARDIS. I like how her relationship with the Doctor grows over the course of Series 4.
Smith and Jones
We meet this season's companion, Martha Jones, medical student, who is the only person in her hospital to remain cool when the hospital is suddenly transported to the moon. Middling episode, though.
The Shakespeare Code
A grand time as we meet Will S. himself, writing his sequel to Love's Labours Lost, called Love's Labours Won. Lots of humor.
Just as the Doctor took Rose to New Earth, so does he take Martha there - only to find that much of the population has taken to the highways, so much so that they live their lives in endless, three-dimensional gridlock. Naturally, aliens are to blame. The Doctor gets to jump from car to car to find Martha before she becomes monster food. I presume the gridlock is an allegory to our congested highways and resource usage, but the episode didn't do much for me.
Daleks in Manhattan/Evolution of the Daleks
The Cult of Skaro has survived their earlier encounter with the Doctor through an "emergency temporal shift" and find themselves in 1930s Manhattan with a plan to integrate human DNA into themselves in order to…well, it's not clear. Make themselves more inventive and thus be able to overcome the Doctor? Add in the Depression, a Hooverville in Central Park, the building of the Empire State Building, and a showgirl in love, and you have a fun episode.
The Lazarus Experiment
An elderly scientist discovers a way to make himself young again - and suffers the consequences. Interesting back-and-forth between the scientist and the Doctor on the downsides of a long life, but ultimately a disappointing episode.
Trapped on board, the Doctor and Martha have only 42 minutes to stop a spaceship heading toward a sun. I liked how the episode unfolded nearly in real-time, but the plot was thin. Another disappointment.
Human Nature/The Family of Blood
In order to hide from a group of short-lived aliens who want the Doctor's body, he uses the Chameleon Arch to make himself a human named John Smith and to forget about his life as a Time Lord; he and Martha then hide out in 1913. John Smith works as a lecturer in a boys' school while Martha works as a maid. Even as he is hunted by the Family of Blood, the Doctor falls in love with a young widow. This is one of the great episodes of the series. At the end, Joan, the widow, tells the Doctor that the seemingly cowardly Smith was braver than he, the Doctor, was: while the Doctor hid from his enemies, Smith was willing to sacrifice his life to make things right.
In 2007, Sally Sparrow investigates a creepy old house, only to find messages from the Doctor from 1969. Another terrific, poignant story featuring the Weeping Angels that explores the nature of time travel.
Captain Jack Harkness returns and grabs hold of the TARDIS as it dematerializes. In an effort to shake him off, the TARDIS travels to the end of the universe, in the year 100 trillion. He, Martha, and the Doctor encounter the remnants of the human race are trying to escape to a place called "Utopia." A Professor Yana is unable to get the engines of the escape ship working. The Doctor helps him. Martha notices that the professor has a pocket watch just like the one "John Smith" had in the episode "Human Nature." Yana is revealed to be the Doctor's old enemy, the Time Lord who calls himself the Master, having used the Chameleon Arch to make himself human to flee the Time War. The Master steals the TARDIS, returning to Earth and stranding the Doctor, Martha, and Jack.
The Sound of Drums/Last of the Time Lords
Following on the previous episode, Jack uses his vortex manipulator to bring the Doctor and Martha to Earth where the Master, having taken the name Harold Saxon, has become Prime Minister. He uses the phone network to control the population and an "alien race" called the Toclafane as his army. He captures the Doctor and uses the Lazarus machine to age him. Martha escapes and, in the next year, travels the world, secretly preparing the human race to use the telephone network against the Master.
* * *
It's hard to recall the subtleties of this season because I've been spending my time watching the Matt Smith seasons. The good episodes stand out, while the so-so ones blend together. Tennant seems to get a fair amount of grief in the fan commentaries, but I rather like his Doctor. Most of the time he plays the role as very human, only occasionally reminding the viewer that, no indeed, he's not human at all. ("The Sound of Drums" most obviously.)