'We've landed in the middle of the river Thames?' shouted Mai, horrified. 'Are you insane?'
'Don't worry.' The Doctor waved a dismissive hand. 'This is February 1814. It's frozen solid. Well, not solid, but as good as.'
Mai stepped through the TARDIS doors, wearing one of the Doctor's old coats and a scarf, and placed a tentative foot on the ice. Then she looked around her.
The Thames no longer flowed - it had frozen into one vast, smooth ice rink. From Blackfriars Bridge all the way to London Bridge, the ice-topped river was crowded with stalls selling food and drink, clowns juggling burning batons, animals performing tricks, kids playing skittles, and hundreds of cold but happy people chattering, laughing and skating to and fro.
'This is the last time the Thames ever freezes over. Of its own accord.' He paused, leaned against the TARDIS, and started to pull on a pair of ice-skates. 'Of course, they did it artificially for the 2030 Dancing On Ice special. Torvill and Dean's final Bolero. My word, they were good...' He trailed off, realising Mai was looking at him as if he was mad. 'Beautiful though, isn't it?'
'I've never seen anything like it.' Mai sighed sadly. 'Where I come from, nothing ever freezes.' She shivered. 'It's so cold.'
'I can lend you some thermal undies.'
'No, I mean colder than it needs to be to freeze.'
The Doctor rummaged in his pocket and pulled out a thermometer. 'Minus seventeen,' he said, noting the temperature.
Mai took it off him. She glared at it for a moment, then looked up, confused. 'It's just dropped to minus twenty.'
'Minus twenty?' He frowned darkly. 'You know what that means, don't you?'
Mai frowned too, concerned.
'Perfect temperature for skating! Come on!' And he slid off across the ice, leaving Mai to lace her skates.
'I've never done this before,' Mai called out to the Doctor as they whizzed through the crowds, past groups of excitable children, grown-ups discussing the weather, gangs of skulking teenagers, donkeys giving rides, and loudmouthed souvenir sellers.
'You've picked it up very quickly,' he yelled back at her. 'Do you like it?'
'It's okay,' she replied, trying to sound nonchalant.
After several minutes the Doctor came to a stop, exhausted but smiling. Mai skated into him and he caught her in his arms. They laughed, stared at one another for a moment, then he released her.
'They had kings and queens back then, er, back now, didn't they?' asked Mai, once she'd caught her breath. 'Who's on the throne?'
'Who's on the throne?' repeated the Doctor, incredulously. 'Mai Kondo! Did you not go to school?'
'Yes, but we didn't do ancient history. Anything before the Great Exodus is a mystery to me.'
'Who's on the throne... goodness... imagine not knowing that...' He trailed off.
'Go on,' egged Mai, cheekily.
'It's gotta be good old Quee... Kin...'
'You don't know, do you?' Mai folded her arms.
The Doctor stopped a man as he skated in their direction. 'Who's on the throne these days, mate?'