Our time gate back home lies through here! So we should be back home in like two minutes, right?
...Well, what do you think?
People Who Threw Away the Will to Live 
The place is lousy with Buggers.
At least there's a nearby Enerton to remind Crono he's still starving after like a week or so.
Otherwise, there's nothing else here besides some dead robot.
There's a FMV for this intro on the PS1 version. I'll add it later once I rip it myself.
It's in bad shape...but it appears to be a humanoid robot! Incredible!
At the Bottom of Night 
I'll make sure it won't. Machines aren't capable of evil...
Lucca, you...pity them, don't you?
A bit of a sore spot for Lucca.
This leads to a robot-fixing montage.
Until, finally...
I'm going to give it some juice! ????'s Theme 
Never doubt Lucca's genius.
Good morning!
Mo......
I'm not your "mistress." I'm Marle!
Just Lucca will do. Impossible. That would be rude.
I understand, Lucca.
R66-Y? Cool! No! That won't do at all!
That's right, this robot is the fifth playable party member. His default name is Robo, so that's what we'll go with!
Crono isn't a very inventive namer.
Your new name is Robo, okay? I am... Robo. Data storage complete.
What... What has happened here? There WERE many humans and others of my kind in this dome.
......It would appear so...
We came through a time warp from the year 1000. While exploring Arris Dome we learned there was a Gate here. We found you when we came looking for the Gate! But the door to the inner chambers is locked, so we're out of luck...
The power is off. If we go to the factory up north, I can pass through security and activate this dome's generator. You'd do that for us? You repaired me. Now it's my turn to help.
Then Marle or I will stay.
I like how there's no option to leave Crono behind.
Anyway, this is the first of many opportunities to choose your party. Crono is always the lead and cannot be changed out. Sometimes your second party member will be set for certain dungeons, like Robo for this next dungeon.
Story-wise, Lucca makes more sense for me due to how the story develops, but I'm picking Marle because she has the more entertaining lines. Generally "best lines" will be how I will choose my party, though I'll try to share any notable stuff other people would have said, or at least show it off in the JERK run.
People Who Threw Away the Will to Live 
You can still swap whenever you want by talking to the other person. On the bright side, party swapping will get a lot more convenient soon enough.
Robo is similar stat-wise to Crono, favoring high Power and Stamina at the cost of low Magic and Evasion. The one key difference between the two though is speed. While Crono is the second-fastest character with a starting Speed of 12, Robo is tied for slowest character with Lucca with a starting Speed of 6.
Robo uses the same damage formula as Crono and Frog for his physicals. He attacks by punching enemies and his weaponry involves upgrading his arms.
Robo does have more variety in his moveset than Crono, and unlike the other three characters, Robo can learn more than two techs right now. In fact, he's about to reach his third very soon.
I'll go into them once I show them off in battle.
Yes, this place is relevant. It's our next dungeon!
Remains of Factory 
We couldn't make any progress before if we tried, but luckily we have Robo to bust us in.
This triggers a battle with an Acid.
Don't let Robo's low damage output mislead you, the metal tank packs quite a...wallop. The issue is more the Acid having max defenses and a resistance to everything besides Lightning, so the only thing that'll do it in quickly is Crono's Slash attack. That said, they only have a measly 10HP, so they'll go down quickly regardless.
They have a decent fire attack, but otherwise aren't really that interesting.
Since Robo and Marle already knew the two techs required for this Dual Tech, their first battle together unlocks Aura Beam. Robo and Lucca don't have any Dual Techs available yet.
This is a bit of a warning for the first section we're doing.
While technically we don't need to do the right side of the factory at all, you'll want to your first time through since you won't know what to do otherwise.
Weird hiding place for an item, up in this room we can't see.
A solid weapon upgrade for Marle.
The trick is to run against this conveyor belt to reach the bottom-left corner of the room. But hitting a robot causes the party to be dropped into some forced encounters. Good luck timing the gaps to avoid these!
*-****-++-
At this point my desk's little keyboard platform fell apart, and I decided to leave the above mid-collapse commentary for posterity. Anyway, the Proto 3 has twice the HP of the Proto 2, but no other differences.
You're then immediately thrust into a fight with another Proto 3 and two Debuggers, which are Buggers but with 20 more HP.
I'll take this opportunity to show off Robo a bit more! Rocket Punch is his base attack, and it essentially does an extra 3/8 of damage on top of his regular physical for a mere 1MP. Not too shabby.
This battle gets him enough TP to unlock his third tech! Unfortunately it'll take awhile to get his fourth.
And this also unlocks his first Dual Tech with Crono's Cyclone.
The last encounter has a whopping five enemies, the perfect opportunity to try out Laser Spin!
It's kinda weak in Robo's hands, only doing (LVL + MAG) * 4 base damage. However, it does this to all enemies and, more interestingly, is our first Dark-elemental attack.
We finally get a chance to rest after that gauntlet, and are conveniently popped out to where we need to go to progress.
The next room has Bugs.
Robot Bugs. They're mechanical-type, you see.
Otherwise, they're bugs. They ram at you.
There's a lot, so let's bust out Rocket Roll!
Rocket Roll does 50% more damage than Laser Spin, using (R_LEV + R_MAG) * 6. Crono's stats aren't factored in here. It's kinda situational, since Crono is generally better off using Cyclone if possible. But it's not too bad if the enemies are spread out, or if there's enemies weak to Dark magic.
Once we get access to the northern section, there's a couple rooms available.
The right room gives us the codes we need to use the crane in the other room.
Triggering this computer also causes the robots to activate and move around frantically.
Cure Beam recovers a simple MAG*14 HP. It's generally better than Marle's Aura, even with Robo's low Magic.
Aura Beam is the same exact thing as Aura Whirl, i.e. casts Aura on everyone. Robo's stats and beefy healing multiplier are unfortunately not factored in.
The crane basically moves two barrels. One is a shortcut back to the entrance of this room, the other to what we need to reach the end of the dungeon.
Also a Bolt Sword if we skipped the Sewer Access earlier.
Anyway, this computer tells us the code we need to progress.
With that, we backtrack and move on to the left half of the factory.
Alkaline are pretty much the same as Acids, except you don't want one of each getting near each other, or it'll trigger a nasty target-all attack. I didn't bother trying to trigger it, though.
Clearing them lets us open the hatch to progress.
While the right room is kinda large and easy to get lost in, the left branch is a lot more linear.
Robo comes with this, but everyone can equip it. It's short for "Titanium".
Even Robo gets a weapon upgrade!
Of course, this triggers another battle with five of these jerks, but after that we can access the elevator and move down to the bottom area.
And right before where we need to go is the last new weapon upgrade, this one for Lucca. This is the first weapon that has something extra, being able to inflict "Stop" on mechanical enemies.
Anyway, with the X A B Y code we can enter this area and take care of business.
This next part is kinda cool, so here's the entire thing in video.
To summarize:
A Shot of Crisis 
Run sequence!
The elevator's shut off, so we're railroaded taking the long way back up.
SILENCE 
Robo's got some help to back us up. Looks like it's our lucky day!
...or not.
Pardon?
At the Bottom of Night 
Have you forgotten our mission? All intruders must be eliminated!
Crono can actually try to get involved, but if so, Robo will reply, "No, stop! These are my brothers!" and continue getting walloped.
This...is not Robo's day.
I hate to see a robot cry.
...And apparently so do the other robots.
I like the subtle moment where Crono steps forward in response. He may not say much, but he certainly doesn't like seeing a friend taking a beating like that, even a robot friend.
Robo's pretty much scrap at this point. Our new robot friend died just as quickly as he came to life.
Boss Battle 1 
They will pay.
Each of these guys has 150HP.
While they're slightly weak to Lightning, Cyclone works much better since it'll hit three at a time. Two Cyclones will wipe a line of them. It's a good idea to give Crono the Speed Belt for this fight so he can easily out-pace the robots.
They resist Fire so it's not worth using Fire Whirl if Lucca's in the party. Marle/Lucca should be on healing duty for this fight.
The R Series have similar attacks to Robo, including RocketPunch.
Also this move.
Revenge is great.
SILENCE 
They trashed him!
Crono, let's take him back to Lucca!If Lucca's in the party, she'll also suggest going back to Proto Dome rather than repairing him there.
Either way, you get a montage of Crono and the other party member slowly dragging Robo back to Proto Dome.
At the Bottom of Night 
The next scene is Lucca repairing Robo, with Marle passed out and Crono leaning from exhaustion.
Shh, don't talk.
I don't know how far we'll get, but that's the plan.
What am I...going to do? Yeah. I mean, what plans do you have for the future?
Robo's Theme 
Robo! You're all right!!
Leave it to Lucca!
Pardon?
That's right, Robo's now a permanent party member. He's got nothing left in this era, and we're the closest thing he has to family now. Why not try to help save the world?
At least now we have a better chance at succeeding with a freaking robot on our side!
Fanfare 1 
About damn time!
That lightning was new.
...I'm sure they're fine.
...Probably.
