Plenty of Doctor Who stories contain events which are referred back to in future episodes. But very few start with the final arc of a character's journey, promising to fill in the rest at another time, and then fulfil that promise quite as fulsomely. So while "Forest of the Dead" has much to say about colonialism, conservation and the rewards of a mundane life, the heart of the story is in the lonely Tenth Doctor's realization that his life will eventually include the companionship he so craves. And the tragedy that it will also end.

(The episode is available on Amazon and iTunes.)

The original title for this episode was "Forest of the Night," but had to be changed once it became clear it would be followed by "Midnight," which was felt to be too similar a name. Steven Moffat suggested "River's Run," which lasted right up until post-production (and initial copies of the US DVD box set of Season 4). Executive producer Julie Gardner wanted it changed again, so Steven suggested "Return of the Dead," before sticking two of the working titles together for "Forest of the Dead."

There's an item of British slang which might need a bit of explaining. When the Doctor berates himself for talking too much, he says "this gob doesn’t stop for anything." Punk fans may be aware that "gob" is British slang for spit. It's also used as a verb, to gob, which describes the action of spitting. However, in this instance, he's using the other vernacular meaning of gob, which is "mouth," as in: "keep your gob shut" and "gobstopper" — the British term for a jawbreaker.

The wedding dress worn by Donna when she marries Lee, inside the virtual reality of CAL, is the same one she wore in "The Runaway Bride." It is not, however, the same one she wore for her final, proper wedding in "The End of Time."

Steven Moffat's original idea for the story contains the germs of two of his best loved Doctor Who stories. He envisioned a library that would contain portals to all the other libraries throughout time and space, which would eventually become the portals in "The Girl in the Fireplace." Also, the original monster menace for this story was a race of stone statues that look like angels, which of course ended up in "Blink."

There's a special significance around the name Cal — supposedly taken from the initials belonging Charlotte Abigail Lux. CAL was also the name of the computer graphics company that created the Doctor Who title sequence used for the Seventh Doctor's incarnation, between 1987 and 1989, including a CGI TARDIS:

https://www.youtube.com/watch?v=d0r27kOw2rw

While the final exchange between the Doctor and River Song has rightly attained iconic status with fans, one of the lines is an echo that goes back to Doctor Who's earliest days. The Doctor claims that "time can be rewritten," to which River replies, "Not those times. Not one line." In "The Aztecs," the First Doctor and Barbara bicker over whether she can alter historical events, an exchange in which he hotly claims, "you can't rewrite history! Not one line!"

On a similar note, the Doctor says CAL's auto destruct mechanism could "crack the planet open like an egg." While a relatively common simile (especially if you start with the word "crack" and work forwards), this is something the Doctor has said before. In "Remembrance of the Daleks," his Seventh incarnation referred to the Imperial Dalek Mothership with a very similar line, saying, "That ship has weapons capable of cracking open this planet like an egg."

The Doctor's muttered line about having 4,022 minds talking inside your own head — "It must be like being, well, me." — could well be a reference to a Time Lord trick revealed in the Fourth Doctor adventure "The Deadly Assassin." They are said to have interconnected minds. There again, it could just as easily refer to the different personalities he has developed after each regeneration.

Donna's children are called Joshua and Ella. They're named after Steven Moffat's son Joshua, and his best friend Ella, who also got to visit the set while the episode was being filmed.

Steven Moffat also has a fascinating theory as to how River Song got her hands on a sonic blaster (or "squareness gun"). The idea is that she found the gun in the TARDIS during her past travels with the future Doctor. It was the same gun that had been used by Captain Jack Harkness in "The Empty Child," and he must have left it in the TARDIS at some point during his own adventures with the Doctor.

NEXT: 10 Things You May Not Know About ‘Midnight’

Now read the rest of the 10 Things You May Not Know About Doctor Who archive.