Mergrave’s shift from institutional loyalty to quiet defiance, culminating in a final act of sacrifice to secure the Doctor’s escape from the collapsing tunnel.

The Doctor uncovers a cryptic note secreted in an ancient book after moving the chevaux-de-frise mirror to redirect sunlight. Sensing danger, he stashes the paper before Mergrave enters to deliver …

Mergrave seizes the moment to ensure the Doctor and his companions escape as the tunnel begins to collapse around them. Knowing Castrovalva’s recursive nature means certain destruction if they remain, …