From TARDIS Index File, the free Doctor Who reference.

Robin Stuart was a friend of Colin Frazer. While vacationing in Amsterdam, they decided to spend a night in the abandoned Frankendael mansion, having arrived a day early and finding no room at the hostels. Colin was abducted by the Ergon and Robin escaped, later getting help from Colin's cousin Tegan Jovanka. They returned to the mansion only to be captured themselves, but were freed by the Doctor. (DW: Arc of Infinity)