By Hanna Rantala and Marie-Louise Gumuchian
LONDON (Reuters) - Four years after wrapping up his second James Bond film, Oscar-winning director Sam Mendes is back, this time on the front lines of World War One in "1917," an ambitious drama inspired by his grandfather's experiences.
Set during a single day in April 1917, the film, presented as a sweeping continuous shot, follows two young British soldiers, Blake and Schofield, who are sent as messengers across enemy territory to stop a dawn attack on retreating German soldiers.
The retreat is actually a trap, and with communications down, the duo are the only hope of saving hundreds of British soldiers from death, one of whom is Blake's brother.
"(My grandfather) fought in the war between 1916 and 1918 and he told us many stories that stayed with me to this day," Mendes told Reuters at the film's London world premiere on Wednesday, attended by Prince Charles and his wife Camilla.
"It's not about my grandfather but it's inspired by my grandfather. He told us one story about carrying a single message across no man's land and that little image kept pulling at me and it wouldn't let me go."
Known for "American Beauty" and "Revolutionary Road," Mendes' last major features films were Bond movies "Skyfall" and "Spectre."
"Even though this was ambitious and tiring in its own way, it's not nearly as ambitious as shooting on five continents and 20 cameras and all that stuff that goes with Bond," he said.
"Even though there were hundreds of people, it was about a war, only one camera, two central characters and a single linear narrative and just one two-hour period in real time of one day in 1917, it felt like a holiday compared to Bond."
The cast, including Colin Firth, Benedict Cumberbatch and Andrew Scott, is led by two lesser-known names: George MacKay and Dean-Charles Chapman, who play Schofield and Blake, respectively.
"I didn't know a lot about the First World War before I started this project so it was a big eye-opener," Chapman said. "It's important that people should remember those that fought in the war ... I had a lot of reality checks every single day."
Reviews have mostly been positive, with critics praising the ambitious storytelling technique in which the camera follows the protagonists as if in one take - a challenge for the cast.
"When you're doing a 10-minute scene and you make a mistake in minute seven, it's really hard, you've got to start again even though you've done stuff you're quite pleased with," Scott said. "It's like walking a tightrope really."