There were a few things that were a siren’s call to the Doctor. Alien invasions, check. Earth in the early 21st century, check. James Lock & Co., Ltd. Ladies and Gentlemen’s Hats & Caps under threat of annihilation? The TARDIS was there quicker than you could say Raxacoricofallapatorius.
"All right!" The Doctor burst out of the TARDIS, sonic in hand. "I’m the Doctor, and we’ll dispense with the normal threats, plea bargains and bribes and go straight to the get the hell off this …" His voice died away as he took in the empty shop. No humans screaming bloody murder. No aliens threatening this, that, or the other. Not a single hat out of place.
Except the one angled on River Song’s curls as she sat on a table, legs swinging as she grinned at him. “Hello, sweetie!”
"Where’s the alien invasion?" The Doctor scanned the area, extending the sonic to check the readings. "Nothing alien here but us."
He eyed her over his sonic. “Did you stage this?”
Her laughter was full of mirth and mischief. “I’m afraid I did!”
He jabbed a finger at her. “River! You can’t just invade hat shops just because you wanted to call me. Some places are sacred. There’s a reason why mobiles exist!”
"But, this was far more effective."
"Still, a hat … oh! Look at this!" He plucked a tweed hat that matched his jacket off one of the racks, plopping it on his head. He spun about. "How do I look?"
"Ridiculous. Hold still, my love."
"Why should I …"
A plasma shot rang out.
"It’s the store’s hat, but …" River’s voice was drowned out as the alarms suddenly sounded. "Well, that’s our cue to go. Come along, Mr. Song."
"I told you to stop calling me that!" The Doctor hissed, grabbing another tweed hat off the rack, shoving it under his jacket and following River into the TARDIS.