The Four Doctors is a Big Finish Productions full cast audio drama based on the long-running British science fiction television series Doctor Who.
It was originally free to subscribers of The Demons of Red Lodge and Other Stories, but can now be selected with any Big Finish subscription.
The Fifth Doctor investigates the Vault of Stellar Curios, where he has observed evidence of time leakage.
The Eighth Doctor also shows up and he and his former self create a time loop trap, spanning between their lives.
This sends the Daleks to the Seventh Doctor's encounter with Michael Faraday in 1854 and the Sixth Doctor's visit to an early Dalek battlefield.