The knock came again, louder. Then the door handle rattled. It was locked, of course. Amy and I regarded the door, and then each other, with a wild surmise.

"It's probably a client," I reassured her, although in truth, paying customers of the Angel Detective Agency (proprietor and sole employee: M. Malone) were few and far between.

"Mrs Williams... Professor Song! I need to speak to you." A male voice.

I replied coolly, "It may have escaped your notice, not being a detective yourself, but this is a detective agency not a university. The sign on the door provides a subtle clue. There are no professors here."

"Well... I need a detective!"

I sighed, rose to my feet, checked my lipstick was on straight, my hair was behaving itself – well, as much as it ever does, which is not much - and my gun was in its usual place, and unlocked the door. Well, it never hurts to be prepared for all eventualities.

The heavyset dark man who entered, wearing a sober black suit and overcoat, had a certain look of familiarity about him. Probably because I'd last seen him not much over an hour ago, chasing me down a White House corridor.

"Mr Stone," said Amy with a polite nod. I was pleased to see she'd recovered some of her old confidence and spark. She's always thrived on a bit of excitement, danger even. I guess it runs in the family – like mother, like daughter. Or do I mean the other way round?

I extended my hand to the newcomer. "I am Melody Malone, and this is my..."

"Mother," murmured Amy.

"... associate, Amy ... Williams. How may we be of assistance, Mr Stone? Unfaithful wife? Lost cat?"

"A detective," he muttered, shaking his head.

"That's me. Melody Malone, hot-shot private investigator. Love a mystery."

"Investigator of... angels?"

"It's just a name," I said quickly. "Why don't we start with how you and Amy know each other?" I glanced at her. She was leaning on the desk, her arms folded.

"I was working in a diner, here in New York," she explained. "Not the best of jobs but things are hard round here, you have to take what you can get. When I... got here... I had nothing but the clothes I was wearing. I was lucky to get a job pretty much straight away. I'd been working there about three weeks when he came in... we got talking. He was curious about my accent. Offered me a job. I thought it was a bit weird, but..." She shrugged. "What else was I going to do?"

"Hmmm." I took a good look at Mr Stone, who wasn't saying much. He looked staid enough, but there was definitely more going on there than met the eye. I wondered what his interest in Amy had been. There was the obvious, of course, but I had a feeling there was more to it than that.

"Bill Stone," he finally introduced himself. "I work for the President... but you know that, of course, Prof... I mean, Miss Malone. That's kind of why I'm here. I have reason to believe President Roosevelt is in danger."

I laughed out loud at that. "So you came to... me? Oh, Mr Stone, I am terribly flattered, but..."

He interrupted. "The danger threatening the President is rather... unusual. And I believe you to be rather an unusual detective."

"No, no, no." I hastened to disabuse him of that alarming notion. "Nothing unusual about me, not a thing. I'm strictly small time. Lost cats, lost husbands – " I shot Amy what I hoped was a reassuring smile "- the occasional misbehaving statue, that's my line. Not endangered presidents. So I'm afraid we can't help you. Goodbye."

I opened the door and invited him to depart through it. He closed it again. I was starting to gain the distinct impression that this guy wasn't buying it.

"Professor River Song." It wasn't a question.

"Stupid name," I commented. "I don't think I know anyone of that name. Do you, Amy?"

"Nope, uh-uh." She shook her head. "Definitely not."

"Like I said –" he went on. "Interesting name, the Angel Detective Agency. Because the matter of which I speak, this threat to the president, does appear to concern... angels."

"Angels?" I was gazing out of the window, watching the street four floors below. I have been known to throw myself from high places – some have even suggested I make a habit of it – but it's always advisable on such occasions to have someone you can rely on to catch you before you hit the ground. And right now, I don't.

"Angels." I turned to face him. His suit looked the part, but the gadget he produced from his overcoat pocket was definitely not 1938 technology. And it was pointing at me.

My new "keeping out of trouble" policy was not going particularly well.