A spoiler-light run through all 15 main story chapters: where each one takes you, which bosses you meet, and the moments you cannot undo.
Remember first: leaving for the orbital elevator in Chapter 9 (Orcal's Testimony) locks you out of Xion, and following the bike signal in Chapter 12 (Burning Xion) locks you out of every other area. Finish side missions, requests, cans and Nano Suits before then, and keep a separate manual save.
Ch. 1 — 7th Airborne Squad (prologue)
A tutorial beach landing.
Boss: Brute — slow attacks, good for learning parry timing.