They have enthralled millions with their exploits in Game of Thrones, the lavish fantasy epic. Now, actors Lena Headey and Iain Glen are teaming up again – this time for a very different film. After starring in a make-believe world of magic, dragons and warriors, they will be seen in a gritty drama focused on the refugee crisis.

In Game of Thrones, which comes to a climax tomorrow, Headey and Glen play the evil queen Cersei Lannister and knight in exile Jorah Mormont. Next they star in The Flood, as immigration officers investigating an asylum seeker, played by Ivanno Jeremiah, who will star in the forthcoming Game of Thrones prequel. Developed over three years, The Flood is one of the first British feature films to tackle the refugee crisis. The film-makers say they aim to show the complexity and humanity on all sides – the refugees, the immigration officers who decide their fate, and the human traffickers.

Headey plays a hardened officer given a high-profile asylum case, in which she must establish why Haile – a young man played by Jeremiah – is seeking asylum after his 5,000km journey over oceans, across borders and through the Calais migrant camp. Glen plays her harangued boss, facing bureaucratic pressures.

Facebook Twitter Pinterest Ivanno Jeremiah features as an asylum seeker in the new drama. Photograph: Joel Frakes

The film was developed after writer Helen Kingston, director Anthony Woodley and producer Luke Healy spent several days in the Calais camp, talking to refugees, exploring the psychological and physical hardship of their journeys. The camp closed in 2016, but hundreds still sleep rough along the northern French coast.

Kingston heard further stories in volunteering with the Refugee Council, teaching English. She researched the routes migrants take and the dangers they face, and watched endless television footage and read newspaper reports. But, in hearing stories first-hand, she was particularly struck by the “truly perilous nature” of their journeys: “When you speak to someone who’s actually been through it, it just hits home that these are ordinary men, women and children… [Also] realising… that there are certain parts of the journey where there are no legal ways to do certain things. So you have to put your life in the hands of criminals and dangerous people.”

The Flood will be released on 21 June, a day after World Refugee Day.