So this isn’t a chronology timeline, but more of a viewing guide on when to watch the TV episodes as they tie together with Torchwood. This is the order I believed they originally aired the episodes and watching them in that order works well.
- Doctor Who S1 Rose
- Doctor Who Christmas Special 2005 Rose
- Doctor Who S2 Rose
- Doctor Who Christmas Special 2006 Donna
- Torchwood S1
- Doctor Who S3 Martha
- “Time Crash” – a mini-episode (only a few minutes) from a children’s fund raiser (BBC’s Children In Need) starring David Tennant and Peter Davidson The Doctor
- Doctor Who Christmas Special 2007 Astrid
- Torchwood S2
- Doctor Who S4 Donna
- Doctor Who Christmas Special 2008 The Doctor
- Doctor Who Easter Special 2009 Christina
- Torchwood Radio Plays (July 2009)
- Torchwood Season 3 (Children of Earth)
- Doctor Who November Special 2009: Waters of Mars Adelaide
- Doctor Who December 2-part Special 2009: The End of Time Wilf
If you read on, I’ll be giving you some spoilers, but I can’t really talk about continuity without talking about it. If you watch them in the above order you don’t need to read on.
I only mentioned the radio plays since they were just released and definitely take place after season two. I’ve listened to the first two and so far it doesn’t seem significant that they need to take place before they should be listened to before the Children of Earth mini-series (no plot ramifications, but they chronologically take place before the 3rd series).
The Sarah Jane Adventures can be watched any time but after her guest appearance in season 2 of Doctor Who would be best (especially if you don’t know who she is) and as long as you’ve started them before the before the end of season four of Doctor Who.
Torchwood Season 1 could be watched any time after the first Doctor Who Christmas episode (but before the end of Season 3 of DW). I don’t think the other Doctor Who specials (or 2009 DW specials) make any difference to watching Torchwood, at least they haven’t yet; but you do want to watch them in order just for continuity of Doctor Who. There are lots of Torchwood references in DW season 2 that might be better if you see them all before watching Torchwood, but I don’t think these tie directly to Torchwood Cardiff (many were just more hints to get you into the new series). Later on, other guest appearances make more sense if you catch them in order, but you’ll probably figure them out, Martha makes appearances on TW and Jack is on DW three different seasons (1, 3, 4) for multiple episodes all at the end of the season. I’m sure someone will disagree with me on some of the significance of some of the references, but I’ll defer back to the original broadcast order listed above.
I think I originally watched parts of Doctor Who season two and caught up on DW season one and saw many episodes out of order and the different actors were distracting, but then I got it all when I finally caught the end of season one and the first Christmas special. This causes me to mix up Christopher Echelson and David Tennant when I think back about past episodes. Also, I think I was watching Torchwood before I watched most of season two of Doctor Who so some of the references came after the fact.
By the time I watched Torchwood, I knew it was the same actor for Jack, but didn’t remember they were using the same character for him. I didn’t even realize it was SciFi at first, the Torchwood pilot seemed more mystical (think of they way the glove worked) until they explained it. Then I realized it was the same guy.
Update: These aren’t all relevant for Torchwood, but it rounds out the David Tennant part of the series (although for that I should add the cartoons and Srah Jane’s Wedding).