Let me see if I can give you a crash course to how plots work in GH2. In GH1 the plots were mostly atomic- everything needed by the plot was contained in one file. In GH2, this is no longer true. You still have a basic plot gear, but it can be expanded by using subplots and persona fragments. Plots are located in the "PLOT_*.txt" files.
A subplot is exactly what it sounds like- it has everything that a regular plot has, can be passed elements as parameters from its parent plot or may request its own. Subplots are located in the "MEGA_*.txt" files.
A persona fragment is a piece of a conversation. These get used in two cases: commonly repeated interactions (such as "goodbye", "nice to meet you", "I have information for you"), and places where you need the conversation to be different depending on circumstances (such as the mission tests, or the battle challenges issued by NPCs). Persona fragments are stored in the "PFRAG_*.txt" files.
The basic Lancemate event is stored in the file "PLOT_LancemateStuff.txt". You'll note that this plot does nothing but select a lancemate and then activate a "*LM_NonComCharDev" subplot. This subplot takes as its only parameter Element 1- the lancemate. To see what it does, we need to go to the file "MEGA_LM_CharDev.txt".
There, you'll find a type of subplot labeled "*LM_NonComCharDev". You can see what kind of a subplot something is by looking at its "Requires" attribute. In this case, there will usually be additional requirements, most of which are of the form "1:xxx", where xxx is something or another. The "1" refers to the parameter passed to the subplot, in this case the lancemate. The "xxx" describes something about the lancemate which must be true in order for this subplot to be selected. Let's look at an example to make things clear.
Scroll down and find the "Let Me Show You Something" subplot. This one requires "*LM_NonComCharDev 1:A.tha 1:TRAIN". The first item, "*LM_NonComCharDev", is simply the subplot type identifier. The next one, "1:A.tha", means that the lancemate's Attitude ("A") must be Thankful ("tha"). For a complete list of attitudes and motivations see the xxranplotgen.txt file or the character development matrix thread here. The third requirement, "1:TRAIN", indicates that this lancemate must be ready to receive training.
This subplot is a bit more complicated than the base plot, but still not very complex. The lancemate will strike up a conversation with the PC, then offer to take them somewhere. Accepting the lancemate's offer activates another subplot, "*LM_SecretPlace". You can find that subplot type in "MEGA_LM_Extras.txt".
The beautiful thing about this setup is that plots are way more flexible than they were in GH1. In this example, after the LM offers to bring you someplace you have no idea what's going to happen next- you could be brought to a junkyard, or to a shrine, or maybe the LM is planning to ambush you. Adding a new possibility is as easy as adding a new subplot. You don't have to recreate everything from scratch. Breaking things into subplots means that each component can be simpler, more stable, and reusable.
This is merely scratching the surface, of course, but hopefully it'll be enough info to get you started. One thing to note- subplots from the "MEGA_CORE_*.txt" files are for the core story, and shouldn't get used in regular plots. Let me know if you have any more questions... and if someone out there could wikify this, it'd be greatly appreciated.
Edited to add- The LM mission plots are all in "MEGA_LM_CharDev.txt", you can c/p the Mercenary or Seeker missions and change the text. Of course it'd help to know the attitude/motivation of your unutilized LM- with XXRANDEBUG turned on, press "`" (the single quote beneath the ~) and it'll give you a list of all your NPC relationships. Find the lancemate and note its "M." motivation and "A." attitude.
