The play is the sequel to the Indian Queen, which takes place in the Incan court, in Peru and which features Montezuma as an upstart Mexican prince and heir to the Mexican throne.

In the Authorial note, "the Connexion of the Indian Emperour to the Indian Queen" Dryden outlines the following character relations.
Zempoalla and Traxalla have one son, Orbellan, and two daughters, Almeria and Alibech

Zempoalla fell in love with Montezuma, but he rejected her love, marrying instead Orazia.

Montezuma and Orazia had two sons, Odmar and Guyomar and one daughter, Cydaria
Twenty years have passed since the narrative time of Indian Queen and the narrative time of Indian Emperour. Orazia is dead and Montezuma is now preparing for another marriage; this time to Almeria.

Summers begins his introduction with a look at Dryden's method, which was to represent probability rather than [historical] truth.  This notion of probability, of course, echoes the arguments made in Aristotle's Poetics.
Summer then launches into sources, particularly Gomara, Cortes, Bernal Diaz, and Juan Mariana.
Authorial use of source material is indeed important--in our reading of Cohen, we likewise noted the sources from which he drew.

The introduction then features the history of various performances; Summers points out that Indian Emperour had a 40 year history.
He also lists the various actors associated with the characters.  Interestingly, several members of the nobility offered an amateur performance, though not necessarily to good effect.
 

The play begins with the epilogue to the Indian Queen.  In this, Dryden sets up the opposition of the Old World and the New.  Montezuma speaks the lines of the Epilogue and seems to be here the voice of the playwright.

The Old World is associated with
convention
Practiced wit
Bounty and generosity

The New World is associated with
Variety
Simplicity, lacking in Artifice
Nature
Naked Indians

The argument is that the Indians would rather have the conquistadors than the practiced and predictable English wit to which, presumably, Dryden objects

Montezuma: Emperour of Mexico
Odmar: his eldest son
Cydaria: daughter to Montezuma
Guyomar: his younger son

Orbellan, son to Zempoalla and Traxalla
Almeria: daughter to Zempoalla and Traxalla; courted by Montezuma
Alibech: daughter to Zempoalla and Traxalla; courted by Odmar and Guyomar

Cortez: Spanish general
Vasquez: Spanish officer
Pizarro: Spanish officer (he went on to conquer Peru in history)

Christian Priest: tries to convert the Mexicans through torture
High Priest: serves Montezuma and dies on the rack

Act I
Association of New World with innocence
Cortez presented as thoughtful, philosophical
Reference to El Dorado (278)
Spanish use Indian guides

Setting: Montezuma's birthday; his court in what is now, loosely, Mexico City
Aztecs are in the temple, sacrificing the flower of their youth
Almeria snubs Mont. and holds him responsible for her parents' deaths 280

Indians view the Spanish fleet 281
The court is to be invaded 283
Meeting between Mont. and Cortez; mention of Charles V
The Catholic Church has given your Empire to Spain; we came here to convert you
Cydaria falls for Cortez and vice versa; forbidden love

Act II
The prophecy of the coming of the Spanish 289
Mont.: I will be true to my convictions until I die
Kalib, a spirit, warns Mont.  You have a chance to control your fate; don't refuse it
Cydaria has replaced, in history, Malinche 291

Cortez says he did not come to conquer 292
Alibech asserts that women will fight 293
Image of man on a horse 295
Cortez and Cydaria declare their love for one another