English: Portrait of Mary Anne Yates (1728–1787), English tragic actress, as Electra in Orestes by Voltaire, chained at the wrist, wearing a pink and blue dress and an ornate powdered grey wig, holding aloft the urn which is pivotal to the story.

This portrait is of the famous tragic actress Mrs Yates in the role of Electra from Voltaire's play Orestes. But although it is a portrait of an actress in a given role, it also emulates the grand society portraits of Sir Joshua Reynolds, the founding President of the Royal Academy in London. Reynolds had pragmatically incorporated his enthusiasm for the Old Masters and classical antiquity into his portraits. Dressing a sitter as a mythological character was common practice on the European Continent, but Reynolds imbued this play-acting with a greater sense of drama, often posing his figures in quotations of poses from classical statues or the Old Masters. When Cotes exhibited this miniature in 1769 he knew it would be seen in the same room as Reynolds's oil paintings. Spurred on by a sense of competition, he clearly hoped to demonstrate in this large work his ability for invention and his virtuosity. Until this time Cotes had always painted exquisite miniatures no bigger than one inch (2.5 cm) high, showing only the head and shoulders of a sitter. This extensive area of ivory has clearly presented Cotes with problems in terms of large areas of surface on which the paint seems dull, and in terms of composition, which seems somewhat stilted.

After a choral ode Orestes arrives, carrying the urn supposedly containing his ashes. He does not recognize Electra, nor she him. He gives her the urn and she delivers a moving lament over it, unaware that her brother is in fact standing alive next to her. Now realizing the truth, Orestes reveals his identity to his emotional sister. She is overjoyed that he is alive, but in their excitement they nearly reveal his identity, and the tutor comes out from the palace to urge them on. Orestes and Pylades enter the house and slay his mother Clytemnestra. As Aegisthus returns home, they quickly put her corpse under a sheet and present it to him as the body of Orestes. He lifts the veil to discover who it really is, and Orestes then reveals himself. They escort Aegisthus off set to be killed at the hearth, the same location Agamemnon was slain. The play ends here, before the death of Aegisthus is announced."