They’re off to deal with a violent lizard-like species of alien. If they don’t stop it, the Earth may end today. Just another day travelling in the TARDIS, in other words.
He’s stopped in the street by a young woman he’s never seen before, but who clearly knows him. That’s not particularly unusual for him. He often meets people out of order. It’s a hazard of time travel.
What makes this day so different, though, is that the woman gives him something to look forward to. After the disaster they’d just been through in 1913, he thinks he needs that.