The history of Beowulf
Beowulf is a long poem that somebody wrote down around 1000 AD. The poem is in Old English, so whoever wrote it probably lived in England. It's one of the oldest poems written in English (but not the oldest).

But the poem tells a story about things that happened in the early 500's AD - nearly 500 years before the poem was written down. The story takes place in Denmark and Sweden, and involves real people who lived in the early 500's AD, who we know about from other written stories in Swedish and also from archaeology.

When Beowulf was being composed and written, the Anglo-Saxons had only recently moved from Denmark and Sweden to England, so they still had a lot of friends and relatives back home, and they told stories about the things these people were doing.