What happened when…?
The original BBC broadcast’s pre-credit sequence featured a reprise of the plot of Aliens of London that led directly into the cliffhanger immediately followed by the resolution of that cliffhanger just before the opening credits.
On Canadian broadcast things happened a little differently. The CBC used the reprise as part of Christopher Eccleston’s pre-show presentation. However, they cut the resolution of the cliffhanger from that and (accidentally, we suspect) neglected to include it the program itself. As a result, the show started with the credits, opening with the cliffhanger already resolved and leaving a great many viewers confused.
So here’s what happened. (Click and scroll across to see the text) The Doctor manages to take off his electrified ID tag saying “Deadly to humans, maybe.” He puts the electrified tag into the Slitheen’s collar unit which creates a short circuit that effects all the Slitheen, including the ones terrorizing Harriet and Rose and Jackie.
Hopefully the CBC will be a little more careful with their editing next time there’s a two-part story!