The 61-year-old actor stars as a doctor in the upcoming zombie-apocalypse movie - which itself is a sequel to Danny Boyle's 2002 flick '28 Days Later' and Juan Carlos Fresnadillo's '28 Weeks Later' in 2007 - and now Fiennes has shared the first two flicks have already completed production.
Speaking with IndieWire, he said: "It's three films, of which two have been shot."
The 'Harry Potter' star detailed the plot of the next movie, which sees director/writer duo Danny Boyle and Alex Garland return to the series after sitting out '28 Weeks Later'.
Fiennes said: "Britain is 28 years into this terrible plague of infected people who are violent, rabid humans with a few pockets of uninfected communities. And it centres on a young boy who wants to find a doctor to help his dying mother.
"He leads his mother through this beautiful northern English terrain. But of course, around them hiding in forests and hills and woods are the infected. But he finds a doctor who is a man we might think is going to be weird and odd, but actually is a force for good."
As well as Boyle and Garland, '28 Days Later' star Cillian Murphy is confirmed to be returning for the upcoming flick in a "surprise" appearance and will not be part of the main cast.
'28 Years Later' - which is slated for release in June 2025 - will also feature Jodie Comer, Aaron Taylor-Johnson and Jack O'Connell, though details about the trio's characters are being kept under wraps.
The following sequel, which is titled '28 Years Later Part II: The Bone Temple', is due to be helmed by 'Candyman' director Nia DaCosta with Garland and Boyle writing the script.