"Things go missing on Heritage. It's one of the hazards of living here. One of the many hazards."
Nobody visits Heritage. Why would they? Dry, dusty and hot, it's nothing but a failed mining colony too stupid to realise that it's actually dead. No-one wants to visit, least of all Ace. But the Doctor's got his hearts set on a flying visit, just while they're in the neighbourhood. That's when he finds out that Heritage wants visitors just as much as visitors want them.
So, while Ace is getting friendly with the locals, the Doctor is trying his best to convince them he's not interested in their secrets. All he wants is a few quiet days and a nice cup of tea.
Trouble is, secrets have a way of unearthing themselves when the Doctor's around. Whether he wants them to or not.