We made our way slowly along the gangway, past my Piccadilly Line platform, and round the rabbit warren which is the King's Cross Underground network. 

As we walked I learnt more about my charge. It was Andrea's first visit to London. She had come down from Carlisle (her return destination had already changed from Manchester to Carlisle) to meet relatives. Who hadn't showed. She declared she would never travel to the capital again. 

As the escalators transported us to ground level, I tried to work out a) how late I was going to be for my friend waiting for me in a pub, and b) whether Andrea suffered from some form of mental illness or if it was a learning disability. 

Above ground I reasoned that the next thing to do was to match Andrea's story against an objective marker: the ticket in her pocket. But that too was confusing: one read "St Albans", another "Manchester". So I searched for the nearest rail representative in a concourse just as hectic as the Underground.

"These are from Euston." 

My heart sank. Andrea looked confused. 

"It's one stop on the Tube," he continued, "or an eight-minute walk. You come out of here, turn left, cross the roa — "

The futility of giving Andrea these instructions was lost on him. I interrupted: "Do you want me to take you there?" She nodded.

It was now that I made my first mistake. I took an executive decision that the short walk to Euston was a better bet than subjecting Andrea again to the chaos of the Tube at rush hour. 

Heading out of King's Cross, I tried to make a rough estimate of how late I was going to be. I called my friend, who offered to pick me up from the bottom of the Caledonian Road. I would drop off Andrea with staff at Euston and rush back. Fifteen minutes max: my next miscalculation. Because even before we reached the British Library, Andrea stopped. "I don't recognise this."

Oh no. 

"Of course you don't," I said as gently as I could. "You've never been to London before."

She stepped back from me, shiftily. 

I whipped out my iPhone and zoomed in on where we were, where we had come from, where we were going. No good. Too much to compute. 

"I can't walk any farther. I'm so tired. I haven't eaten anything." 

And then: "Where are we going?"