The ghost of the castle's first master gunner introduces the opening scene, set in Henry VIII's palace at Hampton Court in 1544. The King flies into a rage when a messenger brings news that the Governor of Portsmouth, Sir Anthony Knyvett, is behind schedule with the building of Southsea Castle and needs more money to pay the workmen. Will Sir Anthony keep his head?

Fortunately the castle was finished when a French invasion fleet came into view in 1545, but the Mary Rose sank on 19 July as she went into battle. The Time Tunnel re-creates the scene at the castle next day as the garrison consider their prospects if the French lay siege to Portsmouth.

Then the ghost guides visitors on to the year 1642 and the English Civil War. The Governor of Portsmouth, Colonel Goring, declared his support for the King. Parliamentary forces besieged the town, and made a night attack on Southsea Castle. In the Time Tunnel we find the commander of the castle, Captain Chaloner, drunk in his bed when the call to surrender comes. He asks if the Parliamentarians will hold off until the morning when he has sobered up, but the castle is captured, with no casualties - except Captain Chaloner's hangover.

The next scene is the interior of the keep in 1759 when disaster struck the castle. Soldiers on manoeuvres on Southsea Common are seen storing their gunpowder on the ground floor, while above women are cooking. Sparks from their fire are thought to have fallen through the floorboards on to the gunpowder, and visitors see and hear the resulting explosion, which claimed 17 lives.

In 1844 the barrack rooms in the Castle were converted to house 150 military prisoners. The ghost takes visitors on to the final scene, where a prisoner has just been flogged for drunkenness, and another soldier is awaiting execution for stabbing an officer with his bayonet. To the delight of the residents of Southsea, the convicts were moved to a new military prison at Gosport in 1850.