Both video game and movie are based on the 1964 Russian scifi novel Hard to be a God by Arkady and Boris Strugatsky. In the novel, Earth has moves forward into the future, while Arkanar is stuck in the middle ages. Earth sends over spies to check up on our once-foes on Arkanar. If I could visit Arkanar as a future Earthling would make my fortune writing national anthems for different kingdoms that were basically just re-written Britney Spears songs.

[Quiet Earth]

It's a tale of two planets, Earth and Arkanar, who live together in brotherly love and all that boring jazz until a bitter war broke out between the two worlds. Afterwards, the Earth ages, matures and develops things like technology, computers, and digital watches. However, Arkanar stays rooted in medieval-era weaponry and warfare and starts stagnating. No one know why until the forces on Earth decide to send a spy to Arkanar, and guess who you get to play? It sounds a bit like Assassin's Creed where you get "regressed" back into the a similar era inside your head, except this time you get to take a blaster, a missile pod, or a hand grenade with you.

This might be the closest that a game has come to sticking you into a Harry Turtledove novel, except you have to also unravel the mystery of Arkanar's situation to boot. Bonus points if your character gets to brandish a laser rifle and tell everyone "This is my BOOMSTICK!"