Roses are Blue, 5.

I jumped to my feet and ran to the edge of the balcony, knowing that I would be too late to save my sister. But as I reached the edge of the balcony I heard, not a sickening thud, but a loud splash.
Looking down below I saw a swimming pool, and I sighed with relief. Scruff surfaced, spluttering. I laughed.
"Hang on!" I called. Stuffing the papers I had taken from the members of the League of the Blue Rose, I hurried down to the poolside.

When I got there Scruff was out of the water, dripping on the paving slabs and shivering. She was sopping wet - but alive.
"Scruff! I thought you were going to die!"
"So did I. I didn't know about the pool," her teeth were chattering.
"Come on," I took her hand. She followed me to the car. I opened the boot and took a towel out.
"You can try to dry yourself with this," I said. "Then we'll go home."
"No! We have to follow them! You heard what he said, 'Little Venice church. That's where they're going!"
"Okay," I laughed. "There's a spare one of my costumes in the boot."
We drove to a secluded spot where Scruff could get changed without the possibility of anyone seeing. She looked kind of odd in a Girl in Grey outfit - she's not really the right shape for it, and she could not get into the gloves and boots. The close-fitting cowl was designed for my bobbed hair, not her waist-length black curls, so she left the cowl down, mostly hidden by her long hair.
Thus almost dressed, Scruff jumped back into the car and I drove off towards Little Venice.
Little Venice is the part of London that surrounds the canal basins of Paddington. The banks of the canals are lined with solid, respectable type villas, while narrowboats ply the canals themselves. It's kind of nice really, and Scruff and I ought to go there more often. We once both got soaked to the skin there in a thunderstorm. We might as well have jumped in the canal!
There was only one church that the League of the Blue Rose could have meant, THE Little Venice church. A marellous Victorian gothic structure of red brick and white stone, with a stump of a tower to one side, it was the Catholic Apostolic Church. It faces the canal, and has been disused for years.

We parked around the corner, despite the fact that Scruff had no shoes on. I wish she wouldn't go out without them, but Scruff is... well, she's Scruff.
Although there were lights on in the clergy house to one side of the great gothic facade, we were able to get over the gate and into the porch beside the tower stump without difficulty. The tools in my belt had the door open in seconds, and we stepped inside.
I had expected dust and decay. Instead the building was beautifully clean. Scruff entered after me, her bare feet silent on the tiled floor.
"Where to?" she whispered as I closed the door.
"Follow me."
We tip-toed into the church, not knowing what we would find.