Despite the fact that Saint Sebastian is the patron saint of the parish of Bascones in Valdivia, the altarpiece is dedicated to the Holy Virgin Mary. The figure of a seated Holy Virgin and Infant occupies the centre compartment on the first section of the altarpiece. Underneath her, a Pietá occupies the predella, sharing it with the representation of the four Evangelists while an Ascension stands on the upper centre section.
Eight reliefs illustrating different passages of the Conception and Childhood of Jesus are distributed over the side sections. These include episodes from the Annunciation, Visitation, Nativity, Adoration of the Magi, Circumcision, Flight to Egypt, Slaughter of the Innocents and Jesus With the Doctors in the Temple.
The figure of the Holy Virgin Mary with Infant in her role as the Mother of Our Saviour is the altarpiece's main protagonist. Her transcendental role in the redemption of humanity through the sacrifice of her only son, of which the figure of the Pietá reminds us, is acknowledged in the Assumption of Mary, the message delivered to us by the Evangelists.