The Doctor had been reluctant to separate with Klára after the close shave with the rogue Solorith; Klára had been just as reluctant to separate with the Doctor, who apparently was still looking a bit unsteady after his brief bout of heart trouble and the rapid aging that had saved her life. However, they agreed to split up for fifteen minutes and if they had no luck, they would meet back at the TARDIS and go to Torchwood for help.
Try to think of anything unusual you saw before the occupation, the Doctor had told Klára, the instructions burning thought her mind as she fiddled idly with the nozzle of one of the spray bottles the Doctor had given her.
It was alarming, how quickly she had taken to trusting this Doctor without a surname, how blindly she had followed his orders, and even accompanied him into that little blue box (that had turned out to be much bigger n the inside than it first appeared) without even considering what could have happened to her. He was a complete mystery, that raggedy Doctor. He was like the moment in the middle of the night, when you felt like you suddenly weren't alone, but weren't frightened; he was the swaying trees in the absence of wind that made a pleasing sound; he was the last and first moment of a life, all the time moving and shaking and flitting about full of nervous energy with such an oldness behind his eyes that he had to be ancient.
Long story short, Klára trusted the Doctor, because he was not the first person to have seen her crying only an hour and a half ago and yet he was the only one who stopped. He dove headfirst into situations that could kill him in two seconds for the betterment of others. He was lonely, like her, and lonely people had to stick together or they'd be lonely forever.
But what had been different in the days before the Solorith invaded? It had been five weeks ago, she remembered that much. Little details, on the other hand, were not her forte; they slipped away from her faster that water cupped in her hands. There had been more homeless people five weeks ago, in the shelter where she lived. She didn't remember their names or faces, but the thickness of the crowds and availability of beds at night had grown much thinner after the Solorith began making their presence known.
After so many of the homeless had disappeared, the screams in the night came to a stop and stray animals stopped vanishing. However, days after that all ended the bodies were found in the forest, bloodless and boneless, swinging from the branches of trees like so many bed sheets hanging on a line. For the weeks after, policemen and politicians looking for re-election had gone on absolute crusades, preaching "justice for the invisible." It obviously wouldn't do them any good, now that the Doctor had shown up. Soon the Solorith would be gone and then she would be gone too, on the TARDIS.
"Hey, watch yourself!" shouted someone from above her head. Klára snapped back to reality too quickly, the nozzle of her bottle snapping off and spilling water everywhere but on the smaller Solorith coming round the corner. As she grappled for the other one tangled in her belt, a splash of water came pouring down onto the ashen monster's back, and as it began to crumble to ash Klára stepped toward it, seeing her moment.
Kneeling down so she was staring into the glowing embers of its eyeless face, knowing she had only seconds, Klára sternly said: "Call your mates. Tell them Earth officially has no vacancy."
By the time she had finished speaking, the Solorith was nothing more than a pile of ash on the pavement in front of her. She looked up at the window and felt her stomach drop at the boy who had dropped the water staring down at her. The hard lines in his face purveyed that he held no pleasure in the sight of her, and the rise of bile in her throat said the same of him.
"We're even," he said before slamming the window closed. Klára sighed and shook her head, checking her waterlogged watch before getting up to find the Doctor.