Prado audio guide,
five stops, free.

It is smaller than the reproductions make it. A hundred and forty-three centimetres tall, eighty-one wide, and most of that surface is nothing at all, a brown-black field with a figure surfacing out of it.

What you are looking at was a wall. Goya painted it straight onto the plaster of his own dining room. In February 1819, at seventy-two, he bought a house outside Madrid on the banks of the Manzanares called the Quinta del Sordo, the Deaf Man's Villa, named after an earlier owner, although Goya had been nearly deaf himself since an illness at forty-six. Over the next few years he covered the walls of two rooms with fourteen pictures that nobody was meant to see. Underneath them were light, pleasant country scenes with small figures. He painted over those.

He gave none of them titles. Everything you have ever heard these fourteen called was decided by other people, and the first list of them was drawn up in 1828, the year he died, by his friend Antonio Brugada. This one was on the short back wall of the ground-floor dining room, to the left of a window, with Judith and Holofernes on the other side of the same window.

Look at the eyes. The whites go all the way round. Then look at the hands, because the white knuckles pressing into the back of the body are, along with the flesh and the blood, the only bright paint in the picture.

Now the argument. Fred Licht has pointed out that none of Saturn's usual equipment is here, no scythe and no hourglass, and that the thing being eaten does not have the body of an infant, or really of an anatomically accurate anything. John Ciofalo goes further: from the legs and the buttocks, he says, this is an adult woman, and she is not struggling because she is already dead and already headless. Whatever Goya was doing, he had drawn the same subject in red chalk twenty-odd years earlier, in 1796 or 1797, without a drop of blood in it.

The murals stayed on those walls for about fifty years and rotted. In 1874 the house belonged to a French banker, Baron Frédéric Émile d'Erlanger, who had them lifted off the plaster and fixed to canvas under Salvador Martínez Cubells, the chief restorer of this museum. D'Erlanger took them to the Paris exhibition of 1878, did not sell them, and gave them to the Spanish state in 1881.

Something went missing in that operation. It has been claimed that the figure originally had an erect penis, and that the detail was taken out during the transfer at d'Erlanger's request.

The panel is 117 centimetres by 162, the first surprise. Everything in it is tiny, and there is an unreasonable amount of it.

Begin in the bottom right corner, because that is the last place in the picture where anyone is still behaving normally. A man is playing a lute and a woman is singing, and neither has noticed the skeleton behind them playing along. Their dinner has just been overturned: cards and backgammon board on the floor, the diners drawing their swords, the court jester crawling under the table.

Now let your eye travel left and up, and watch the painting stop caring who anybody is. Peasants, soldiers, nobles, a king and a cardinal are all being collected. The cardinal is escorted by a skeleton wearing his red hat. The king still reaches for his barrels of gold while one skeleton loots them and another holds an empty hourglass in front of his face. Under the wheels of the death cart a woman has fallen with a thread in one hand and scissors in the other, a moment from cutting it, which is Bruegel painting Atropos without announcing her. The living are herded into a coffin-shaped trap decorated with crosses, and in the top left corner skeletons ring the bell for the end of the world.

Pieter Bruegel the Elder made this around 1562, in oil on oak, welding two traditions together: the woodcut Dance of Death from the north, and the Italian Triumph of Death of the kind painted in the Palazzo Sclafani in Palermo and the Camposanto at Pisa. He owed a great deal to Bosch as well, whose hell is the next stop on this page, in room 56A.

It spent fourteen months in this museum's workshops before going to Vienna for a Bruegel exhibition in 2018. José de la Fuente stabilised the oak boards; María Antonia López de Asiaín worked on the paint. The cleaning handed back things earlier restorers had removed, including a small plate on the dinner table and part of the belt of a green dress, and it brought the reds and blues up along with the reflections in the water. It also showed how he worked: to make smoke look transparent he took paint back off the panel, some of it with the pressure of his fingers, and he left the marks of the stick he rested his painting hand on.

One more thing to hunt for, since it dates the scene. Among the everyday objects scattered through it is an early mechanical clock. Bruegel put the newest technology of his century into a picture about how little any of it is going to help.

The room is the upper floor of the central block Juan de Villanueva designed, and it is arranged around one canvas: 318 centimetres tall, 276 wide.

Velázquez finished it in 1656, and nobody called it Las Meninas then. The early inventories list it as La Familia, The Family; this museum's catalogue printed the title everyone now uses only in 1843. Almost everything we know about who these people are rests on one text, a description published in 1724 by the painter Antonio Palomino, sixty-eight years after the event.

So, from the middle outwards. The child is the Infanta Margaret Theresa, five years old, at that point the only surviving child of Philip the Fourth and Queen Mariana. Kneeling to hand her a drink from a red earthenware cup on a gold tray is María Agustina Sarmiento de Sotomayor. The other lady-in-waiting, caught at the start of a curtsy, is Isabel de Velasco. On the right are two court dwarfs, the Austrian Mari Bárbola and the Italian Nicolás Pertusato, who is nudging the mastiff with his foot. Even the dog has a pedigree: it is thought to descend from two mastiffs sent from Lyme Hall in Cheshire to Philip the Third by James the First of England in 1604.

Then the two difficult figures. At the back, in the lit doorway, is José Nieto Velázquez, the queen's chamberlain and head of the royal tapestry works, and possibly a relative of the painter. He has one knee bent and his feet on different steps, and nobody has settled whether he is arriving or leaving. He is also standing precisely at the vanishing point of the whole perspective. Every line in the room runs to a man who may be on his way out. And in the mirror on the back wall are Philip the Fourth and Queen Mariana, the only double portrait of the two of them Velázquez is known to have made.

Two things you can check with your own eyes. The red cross of the Order of Santiago on the painter's chest was not there when he put his brush down. He was not admitted to the order until 1659, three years after this was finished and a year before he died. Palomino says the king had it painted in afterwards, and that some said the king did it himself. Second, the infanta's left cheek. In 1734 the Alcázar of Madrid burned down with this canvas inside it; the court painter Juan García de Miranda repaired the damage, and that cheek had to be almost entirely repainted. The canvas was cut down on both sides too, so the room you are looking into is narrower than the one Velázquez painted.

It has confused people from the beginning. When the royal collection was inventoried after the fire, in 1747, the infanta was written down as her older half-sister Maria Theresa, and the mistake was copied again in 1772. The last cleaning, in 1984, was done by the British conservator John Brealey, and the Spanish press went after him daily on the grounds that only someone born and raised in Spain should touch it; one day a small riot broke out outside the room where he was working. Federico Zeri's verdict was that people objected not because the picture had been damaged, which it had not, but because it looked different.

One last fact. It has left this building once, for a Geneva exhibition in 1939 when the civil war emptied the Prado. Since then its size, its importance and its value have kept it here, so this wall is effectively the only place on earth where this can be done.