Bosch: Oh! That's it!
I can repair the Grandleon as
long as I have that Dreamstone.
But the Grandleon and the
Dreamstone, where on earth...?
Oh, never mind.
Wait a minute.

Everyone goes downstairs. Corridors of Time plays. Crono can walk around and talk to Bosch and his assistant.

Bosch walks over to a bookcase and starts reading.

Bosch: I'm looking something up now.
Don't speak to me.

Bosch finds what he was seeking.

Bosch: In order to repair the Grandleon,
it is essential to not only refine the
Dreamstone but also make the sword activate.
You refine the Dreamstone for me.
I'll carry out the activation of the sword.

Lucca: Got it!
Leave it to the great Lucca!

Bosch: In order to activate this sword, I must
breathe life back into the sword's substance
now that it has broken and lost its power.

Bosch: The spell to make the sword
activate is a highly difficult one.
Would you not speak to me for a while?

Lucca: Umm, this is a lot of trouble.
You're distracting me, so go away!

Robo: We lack the equipment, SO it seems
the purification WILL take some time.

Bosch: All right, I've finished.
Would you bring me the Dreamstone?

They begin. There's a bright light and a shrill noise.... The scene cuts to back upstairs, with Memories of Green playing as usual.

The lights dim as the character who assisted Bosch holds up the repaired sword.

Bosch: Behold!
THIS is the Grandleon!

Marle: AWESOME!
This is that Grandleon!?

Lucca: This is the Grandleon....
Energy unknown to me is coming from it.
It arouses my desire to investigate....

How can Lucca respond here if Bosch's helper doesn't speak and Lucca always volunteers to be Bosch's helper if present? Start the scene with Marle and Robo in the group, then swap Marle out for Lucca downstairs. Sneaky. Also, the NA version translation bugs me, and not just because it uses "it's" instead of "its". Does it make much sense for someone who has been using magic for some time to speak of being skeptical of the supernatural?

Robo: It IS mysterious.
I CAN sense spiritual energy
from this Grandleon itself.

Bosch: Well, that's just how things
go once you get me involved.

Marle: Well, Crono, let's bring the
Grandleon to Frog-san, quick!

Lucca: Well, Crono, we're bringing
the Grandleon to Frog, quick!

Robo: Well then, Crono, LET US
go to hand the Grandleon
over to Frog-dono.

Bosch: A sword may be either a means
of taking life or a means of saving life,
depending on how the user's heart is.
Be sure not to misuse it.

Frog is still here, as gloomy as before.

The party shows him the sword. The music stops dead.

Frog: It can't be, this sword...
is the Grandleon...!?
...let me think this over.
You can rest here tonight....

Fade out, then fade in to a flashback, in Guardia Castle. Courage and Pride plays. An unfamiliar knight stands in the throne room.

Leene: I see you're going.
Cyrus....

Cyrus: Yes. I've been thinking it's about
time someone retakes that badge from
that Golden Frog character....
I also want to try confirming the so-called
legendary sword with my own eyes....

Guardia: Cyrus, you are an
indispensible man to this country....
And also an irreplacable friend
to Leene and myself.
Be sure to come back.

Cyrus: Certainly, as long as I have life.
Even should something happen to me....
Well then, I take my leave....

Cyrus leaves the throne room, and Leene stands to watch him go. An assembly of soldiers awaits him outside.

Soldiers: Cyrus-sama!!
We, the Royal Knights, all of us!
We are all praying for our
leader's safe journey...!!

Cyrus: ...you guys.
Take care of things.

Cyrus: Sorry to keep you waiting.
Now, shall we go?

Leene: Glenn!
You be careful too.

Glenn: May the Queen also
remain in good health....

The scene changes, and Battle plays. Cyrus, weilding a red sword, fights off monsters in a forest, along with Glenn. The scene changes again, and Boss Battle 1 plays. Now, Cyrus, accompanied by Glenn and an unknown young woman, faces a giant frog wearing a crown and flanked by two smaller frogs.

Cyrus: Naturally, allow me to do so.
Here I go, G-Frog!
Nirvana Slaaash!

Cyrus jabs at the frog several times quickly. It shrieks.

The frog tosses the badge to the ground and leaves with the lesser frogs. Cyrus takes the badge. Scene change to an unknown, dark location. Wind howls, and we hear sounds of combat.

More fighting noises, and a sound like something metal breaking.

Glenn: Cyrus!
The sword...!?
The Grandleon...!!

The screen scrolls up and lightens enough to show Cyrus and Glenn standing on a mountainside, facing Vinnegar. Behind Vinnegar stands a man with pale skin, long pale blue hair, dark clothes, and a navy blue cape that billows in the breeze. He seems to be paying no attention to the fighting behind him.

Vinnegar: Kyahaha, what's the matter,
are you done already?
Now that the legendary sword's
broken, I doubt you can do a thing!!

Glenn: C, Cyrus....
I, I can't....

Cyrus: Listen, Glenn.
I'll hold them off.
Run away then, even if by yourself.

Cyrus: At this rate, we'll both
be done in....
Go, Glenn.

The mysterious figure finally turns around. He drapes his cape around himself, and I think he looks a lot like a vampire.

Magus: Such composure, Cyrus, was it?
Is this any time to worry about another...?

Cyrus: All right, Glenn?
Here I go!!
Uoooooh...!

Cyrus charges Magus, who casually fries him.

Cyrus: R... run away....
Glenn....
The Queen....
Take care of....
Leene-sa....
......

Cyrus bursts into flames and disappears. 3-05 ジール宮殿 (Zeal Palace) starts playing.

Glenn: C... Cyrus!?
Cyrusss!!

Magus: Hmph, what's wrong...?
Aren't you attacking?

Vinnegar: Gyoheheh...
Like a frog caught in a snake's glare.
Youngsters these days.
How about it, Magus-sama?
Changing this coward into a
suitable form?

Magus: Hmph, very well....
I abolish all those who block
my way, without exception.

Magus gestures, and a bolt of energy strikes Glenn. He starts glowing weird colors.

Glenn: U... uwaah!!
Guaaaaa...!!

Glenn falls off the cliffside nearby.

Vinnegar: Gya, hahahaha...!
You damn coward worm...!

The scene changes back to the present, in Frog's dwelling in the woods. Everyone is asleep but Frog. At the Bottom of Night plays.

Frog: Has it already been
10 years since that...?
Can I do it...? Me...?
Cyrus....

The screen fades to black, and the music stops.

The screen fades in, showing everyone now awake. Frog's Theme fades in.

Frog: I don't know how much I'll be
able to do.... But let's try going.
To Magus Castle....
He's strong....
Are you ready...?

Frog joins the party. His equipment and skills are the same as before, but he's gained some levels. It's time for a few side trips, and not just to equip Frog....

Spekkio said to bring other people, so the party takes Frog here.

Spekkio: This's another weird one
you've brought.
As you might guess given this guy's
a frog, he's got "Water" power.
Hanyaharahettamitaai!!

Frog flexes his muscles as water splashes around him. Fanfare 3 plays.

Frog immediately learns ★ウォーター (Water), and, like the others, can now continue learning techniques normally.

As usual, Gina is here

Gina: .........
Crono....
We can't keep such a large
pet in our home.

Probable pun: 蛙 (frog) and 飼える (able to keep a pet) are both read "kaeru"....

Frog: G... good lady.
I am not a pet. Though my form may
be like this, I'm a full-fledged soldier.

Gina: Oh, I'm sorry, Soldier-san.
I could have sworn....
I'm truly sorry, Soldier-san.

The way it goes all katakana seems to suggest she thinks ブジン (bujin = soldier) is his name. This makes some sense, since it is an older and less common term for a soldier (or more literally, a "military man" as opposed to a "scholarly man"), so it's understandable that she wouldn't make the connection.

Frog: ......
Soldier is good enough....

Taban: The great Taban's great invention.
The perfect armor for Lucca!
Use it well!

Magus's army is said to come through a cave here, but there's nothing visible but a plain cliff. Frog steps forward and pokes at the cliff for a bit, then At the Bottom of Night starts playing, and the screen fades into another flashback. In a forest, four young boys dash around a fifth, who is spinning uncontrollably.

An older boy runs up, scaring the four who are running around.

The four of them run away, leaving the fifth still reeling. Cyrus steadies him.

Cyrus: Look, Glenn, a man's got times
when he has to fight back, too.

Glenn: But... getting hit hurts.
Even for them....

Glenn's saying that even they hurt when you hit them, not that it hurts when they hit him.

Fade out, and fade in to a bridge. A man stands there, looking out at the sea. A young man, who looks the same as Glenn from the flashback in Frog's dwelling, walks up to him.

Cyrus: Hey, Glenn...
I'm thinking I'll join the Knights.

Glenn: I knew it!
I'm sure you can become a great knight.
I guarantee it.

Cyrus: Really.
I think your sword skill is the better.

Glenn: I'm hopeless.
I'm sure I'd be scared stiff in a real battle.

Fade out, then fade in to the mountain where Glenn faces Magus and Vinnegar alone after Cyrus's death.

Magus gestures, and a bolt of energy strikes Glenn. He starts glowing weird colors.

The scene fades out as Glenn nears the cliff edge, then fades back in to show Frog, washed up on a riverbank lower on the mountain. Something round and shiny drifts down the river and stops in front of him.

"Badge...
...the... Hero's....

The...Medal.
...The Hero's...Medal...

The scene fades out, and we're back to the present. Frog turns around.

Crono dramtically stabs the sword into the ground (note that you shouldn't actually do that with a sword, as it can dull the edge) and walks out of the way. Frog steps up to it, looks down for a moment, then looks up again, laughing froggily. Frog's Theme starts playing.

Frog: My name is Glenn!
Cyrus's wishes and aspirations,
and this, the Grandleon....
I here and now inherit them,
and will strike down Magus!

Frog takes the sword by the hilt, and lightning crackles around it. He pulls it out and holds it aloft, and energy surrounds him and flares from the tip. The pyrotechnic display fades, and he waves the sword around. With a crackle of lightning and residual explosions, he slashes down the cliff wall, and it slides aside, revealing a cave. Debris falls in the aftermath. Frog gestures dramatically at the cave with the Grandleon, and the party enters. Frog now wields the Grandleon, which is powerful enough that his attack hurts more than Crono's or Robo's, despite his comparitively low strength. When wearing the Hero Badge, he gets critical hits more often, meaning he does even more damage. Underground Waterway plays in the cave, where a bat follows them around, fluttering away whenever there's a battle, then returning afterwards. The monsters are resistant to normal attacks, but fall easily to magic.

Frog: Could what you're using
be what they call magic?
I heard none could use it
now but the Demons.

Marle or Lucca will respond with this line...

"If we go the Farthest Reaches of Time,
you might become able to use it too.

...and Robo with this one.

Robo: If we go to the Farthest Reaches of Time,
you MAY become able to use it as well.

The party passes through the cave, to find themselves on an island with a large castle deep in a dark forest....