Ross Poldark (played by Aidan Turner), Sam Crane (Tom York) and Dwight Enys (Luke Norris) race to the rescue when blasting in the mines causes a disastrous flood.
The episode will also see Ossie Whitworth (Christian Brassington) plan to have Morwenna (Ellise Chappell) committed to an asylum, and Emma Tregirls (Ciara Charteris) has to answer for Sam.
Ahead of the show, Lady Whitworth actress Rebecca Front has spoken out about playing the mother of cleric Osborne, and whether she is to blame for her son’s behaviour.
The Thick of It star told Radio Times her new character is “ghastly”.
She explained: “Money is a problem but she’s probably the poshest person in most rooms.
She wouldn’t have been a hands-on mum
Lady Whitworth star Rebecca Front
“We reckon this is all ancestral jewellery. Money is not available but this is there so she’s wearing a fortune which is great for me.”
Preparing for the role, the 54-year-old actress delved into the research and background of her character.
Rebecca continued: “They may not have the most money and so they may not be the poshest now in terms of what they have and own, but historically they probably are and more importantly, they consider themselves the most aristocratic.
“The fact that her son is a clergyman and has to earn a living shows their financial status. Joining the church is a younger son sort of thing to do and we don’t know if there is another child, there doesn’t seem to be.
“But there’s a sense that ‘we’re better than this’ with them.
BBCPoldark spoilers: Lady Whitworth star Rebecca Front speaks out on Ossie Whitworth future