The reaction begins with a hydride nucleophile reacting with the ester carbonyl carbon to form the tetrahedral intermediate.
The carbonyl reforms to produce an aldehyde with the loss of the alkoxide ion.
The resulting aldehyde undergoes a subsequent reaction with a hydride nucleophile to form another tetrahedral intermediate. The carbonyl is not able to reform, because there are no stable leaving groups.
Therefore, the tetrahedral intermediate is protonated to produce a primary alcohol.
