Angels bearing the Instruments of the Passion appear to Christ at prayer. The disciples sleep. In the background Judas comes with soldiers to arrest Christ. The dead tree and vulture may indicate death. New growth and the pelicans are perhaps hopeful signs for the future. Jerusalem (which is represented here as a walled city), and which was then under Roman rule, includes an equestrian statue, a column with relief sculpture, and a theatre like the Colosseum, all inspired by monuments surviving in Rome.
The other version of this picture by Mantegna is in Tours (Musée des Beaux-Arts) and is firmly dated to 1457-9, although the composition is in reverse, and the disciples differently arranged.
The National Gallery picture is probably a little later in date. It is more fluent as a composition and more dramatic in presentation, with a more effective relationship between figures and landscape. The putti hold instruments of the Passion.
The picture may be related to a drawing by Jacopo Bellini (London, British Museum). There is also a version (in the National Gallery Collection, 'The Agony in the Garden') by Giovanni Bellini which is influenced by Mantegna's picture.