History-meets-mystery as Codex uses family entertainment to unlock our ancient past. Each week 5 members of the public spend the night at the British Museum in London among its unrivalled collections of historical artefacts. Their challenge is to decipher the "Codex" – a riddle written in a mysterious code.

To ‘crack the Codex', the team follow a trail through the Museum, from the moonlit Great Court through the galleries to the Mummy stores and other spooky places. En route they visit 5 major Artefacts on the theme of the week, and tackle a historical challenge directly related to each one. If they complete a challenge successfully and in time, 2 symbols in the Codex are turned into letters. As well as earning letters, the team is also whittled down in a series of Head-to-Head challenges, also on the theme of the week.

After 5 Artefact Challenges, the sole surviving player returns to the Great Court, where the rest of the team – and the viewers – attempt to ‘crack the Codex' and decipher the riddle. Finally, the surviving player enters the world-famous Reading Room and must use the riddle to decide which of 5 similar artefacts displayed is the only one to which the riddle refers. If the finalist chooses the correct artefact, the team win the show, and a trip to a site of historical interest.