So the Doctor sonics the lock on the door, which is funny considering that it's an open establishment, but I don't know what things were like at the time this story takes place, and he spends the whole episode coming in through back doors, so who knows. That's not the point; the point is the creepy kid's voice that catches Rose's attention: "Mummy? Mummmmy?" ("Mummy," if you didn't know, is British for "Mommy." Weirdly, they call reanimated Egyptian Pharoahs "Mortimers" over there. I don't get it either.) The Doctor cracks the lock on the bar and tells Rose to come along, but she's looking all over for the child, who keeps calling. Finally, we see him on a rooftop, backlit, scary as hell, wearing a gas mask. Short cuts get us closer and closer to him, the white circles of the eyes. It's compelling and awful. Rose calls to the Doctor and says that there's a kid on the roof, but he's gone. She takes off to save the kid, looking for a way up the building, as the child watches. Rose, the thing is? This is objectively creepy. You can tell by the backlighting.
The Doctor heads into the club, where a lady is singing and the usual '40s kinds of people sit around with long cigarette holders and minks and whatever. Clue #1. Well, or #2, if you count the gas mask, but that could be any old kind of creepy. "For nobody else gave me the thrill/ when I have found I love you still/ it had to be you, wonderful you..." The only place you can get milk is Earth; the only person who can save the Ninth Doctor is Rose; this kid only has one Mummy, and Rose isn't it. It had to be you.