UAC scientists have discovered another portal to Hell. There's an isle of death on the other side. An infiltration team has been sent through to clear the area, but they haven't reported back … you're being sent in to discover what happened, and clear the area for further teams.

https://www.doomworld.com/forum/topic/119752-third-map-isle-of-death-doom-ii/?tab=comments#comment-2258839

An ancient medieval citadel has been discovered under the Welsh Mountains, with help from documents obtained from the Mars Base during the first invasion of Hells forces. A team of archiologists has been sent in to learn more about the connections between the ancient martian civilisation, and medieval Britain.

But contact with the archaeologist team has been lost … you’ve been sent in to clear the citadel.

https://www.doomworld.com/forum/topic/119397-new-doom-ii-map-the-halls-of-aethelwulf/?tab=comments#comment-2251069