The Mansion is one of the largest buildings on the peninsula, the other being Blackhall Manor. The mansion itself, while being in excellent condition compared to buildings in the Capital Wasteland, is still in bad shape. A good amount of the wood is rotting, and parts of the floor will break if you step on them.
The mansion bears the name Calvert from its former inhabitants, the powerful Calvert family. The Calverts were a major force in pre-war Maryland, spawning a political dynasty which saw family members elected to various offices at both the state and national levels.
Calvert Mansion is inhabited by a ghoul named Desmond Lockheart and his two dogs, Freki and Geri. He has been attacked numerous times by a group of tribals, whose aim is to raze the mansion to the ground. According to Tobar, the building holds hidden treasures.
The place is filled with ammunition and medical supplies set up by Desmond that can be used to survive the tribal attack. In the numerous wardrobes/cupboards are pre-War outfits, hats, and bonnets.
Inside Desmond's corner is an assortment of weapons, ranging from a sledgehammer to a flamer, as well as a punga experiment that will illuminate a light bulb.
The Panic Room is a room under Calvert Mansion that was designed to be a shelter in the case of an attack. It can only be accessed after the quest Thought Control. Desmond can be found in the panic room if you completed Thought Control in his favor. Inside are numerous items and containers. There are 2 ammunition boxes, a cabinet, a desk, a filing cabinet, a few shelves, and 5 television sets.
In the fight against the invading tribals, Desmond will make otherwise inaccessible rooms accessible but only temporarily. If you wish to loot the rooms, you should store that loot in the main room with the staircases. Calvert Mansion is eventually made totally inaccessible though, by being completely destroyed, but only upon completing Thought Control.
After the explosion, the quest map marker leads to a small hatch in the ground that leads to Desmond's Panic Room. Desmond is in there grieving about the explosion killing his 'pups', Freki and Geri. The room itself is filled mostly with food and ammunition, including a miniature Nuka-Cola Quantum.
If you walk around the ruins of the mansion, you will find a few pieces of rubble. Several are found on the cliff behind the mansion, with a piece stuck in the tree, a piece down the cliff, and one on the island the lighthouse is on.
To the west of the mansion is an atrium with a couch, television set, makeshift bed, storage containers, and a hidden Chinese bunker. Entering the bunker requires the cryptochromatic spectacles (see The Velvet Curtain) or exploits. This portion of the mansion grounds remains intact after the explosion, providing a unique vantage point to watch flaming debris. This area can be used as player housing.