BEHOLD, O good and sweetest Jesus,The indulgence is just icing on an the cake. It's a beautiful prayerful meditation on the Cross anyway.
I cast myself upon my knees in Thy sight,
and with the most fervent desire of my soul
I pray and beseech Thee
to impress upon my heart
lively sentiments of faith,
hope and charity,
with true repentance for my sins
and a most firm desire of amendment:
whilst with deep affection and grief of soul
I consider within myself
and mentally contemplate Thy five most precious Wounds,
having before mine eyes that which David, the prophet,
long ago spoke in Thine own person concerning Thee,
my Jesus: “They have pierced My hands and My feet,
they have numbered all My bones.”
Update: As Ikedi notes in the comments, the usual conditions for the granting of a plenary indulgence apply:
N20. §1. To gain a plenary indulgence, in addition to excluding all attachment to sin, even venial sin, it is necessary to perform the indulgenced work and fulfill the following three conditions:
sacramental confession, Eucharistic Communion, and prayer for the intention of the Sovereign Pontiff.
§2. A single sacramental confession suffices for gaining several plenary indulgences; but Holy Communion must be received and prayer for the intention of the Holy Father must be recited for the gaining of each plenary indulgence.
§ 3. The three conditions may be fulfilled several days before or after the performance of the prescribed work; it is, however, fitting that Communion be received and the prayer for the intention of the Holy Father be said on the same day the work is performed.
§4. If the full disposition is lacking, or if the work and the three prescribed conditions are not fulfilled, saving the provisions given in Norm 24 and in Norm 25 regarding those who are “impeded,” the indulgence will only be partial.
§5. The condition of praying for the intention of the Holy Father is fully satisfied by reciting one Our Father and one Hail Mary; nevertheless, one has the option of reciting any other prayer according to individual piety and devotion, if recited for this intention.