The man to thank for the almost unbelievable cameo is - wait for it - Jonathan Ross. No, really. Here's star Martin Clunes himself explaining all in a recent appearance on Wossy's chat show:
"I came on here to plug [ITV drama mini-series] Arthur and George and Sigourney Weaver was here, who I'd never met.
"She said, 'Oh I've watched every episode of Doc Martin...'
"It turns out, she is best friends with [Clunes' Doc Martin co-star] Selina Cadell - they were students together - who plays Mrs Tishell, our chemist and so that's why she'd been watching."
Playing an American tourist visiting Martin's Cornish town, Weaver jumped at the chance of a small role.
Twitter users, however, just jumped of their skins.