XIII is almost a living comic book - you interact with characters and move the story along, but there are cut scenes and even in-game interactions which are right out of the pages of a comic book. It's quite fun! Now unfortunately I didn't get the whole walkthrough up while it was fresh in my mind. So the first half of the walkthrough is done normally - but the second half is just my raw notes, since I don't remember it well enough now to 'embellish' it with details. I hope it still helps out!

The Basics
The Characters and Actors
The Keys
The Skills

The Walkthrough
On the Beach
Getting To the car
Winslow Bank
Escaping Winslow Bank
Escaping from the FBI
Following Major Jones
Penetrate the Base Enclosure
Infiltrate the Base
Set General Carrington Free
Emerald Military Base Rocky Mountains
Interrogations Birds Eye Valley
Kellownee Lake - Rocky Mountains
Plain Rock Penitentiary
Find Way Out of Asylum
Jones and Airplane
Calaveritan Island, Mexico
Colonel McCall
Sabotage SPADS Camp
Infiltrate the Submarine
Get To the Radio
Use the Torpedo Room
Open the Floodgates
Find Carrington
The FBI
Willard's Estate
Clan of Twenty
Spy On the Meeting
Get To the Chopper!
Walter Sheridan Estate Washington
Neutralize the GIs
Stop Operation Total Red
Cancel Base's Self Destruct
Revelations