This cast is taken from a large marble Roman relief now in the Capitoline Museum in Rome, dated to the second century AD. It depicts the culmination of the ancient Greek legend of Perseus and Andromeda. Queen Cassiopeia, mother of Andromeda, claimed that she was more beautiful than the Nereids, thus angering the sea god Poseidon. Poseidon sent a sea monster to attack her land as a punishment. To avert the danger Andromeda was chained to a rock in the sea as a sacrifice. This scene shows the moment when the Greek hero Perseus, seeing Andromeda chained to the rock, comes to her rescue, slays the monster, and holds out his hand to guide her safely to shore.

The original Roman relief was one that many artists and Grand Tourists of the 18th century were inspired by.This cast belonged to the sculptor John Flaxman and its somewhat linear style has echoes in his work. A miniature copy of this relief, in ivory, is inset into the front of the celebrated 'Walpole Caibinet', now in the Victoria and Albert Museum, which was commissioned in 1743 by Horace Walpole on his return from the Grand Tour to incorporate a series of ivory plaques, most representing classical authors or derived from antique gems. The cabinet was displayed in the Tribune at Walpole's country residence, Strawberry Hill at Twickenham, from the 1760s. The Royal Academy, London, has a full-size cast from the same relief, presented by Charles Townley in 1794.

This cast was formerly in the possession of John Flaxman and was acquired by Soane in c.1834 when he was invited by Flaxman's sister-in-law Maria Denman to select items from those left in the sculptor's studio after his death (he had died some years earlier in 1826).