This episode from Doctor Who: The Fourth Doctor Adventures Series 10 Volume 01 is available free to Big Finish Newsletter subscribers January 2021.
10.1 The World Traders by Guy Adams
When a day out in 21st Century London leads to the TARDIS being impounded, the Doctor and Leela find themselves getting involved with the mysterious Amapan Investments, a company that’s managed to become remarkably successful in a remarkably short amount of time.
So what’s their secret? The Doctor and Leela are about to find out.
The Usurians have come to Earth. And they have big plans.