The Return to Mermaid Caves is an optional mid-game dungeon accessible after successfully clearing the the initial Mermaid Caves. To return, speak to Marianne in the morning and ask about the mermaids. She will then want to send the Cutie Knights on a mission to rescue the captured Magical Girl, and kidnap the mermaid's leader to force them to surrender. Eric(a) will then ask for a while to think about it. Talk to Marianne again later to begin.
Similar to the original visit to the location, this is a one-time dungeon that cannot be repeated. Furthermore, any loss here will result in a game over, so be very sure you are prepared before coming here, and preferably leave a save file from before you enter. Losing against most enemies, including the boss, will result in the Mermaid Sex Slave Game Over. Losing against the Nereid enemy though results instead in the Beach Babes Game Over.
Outskirts
After arriving in the mermaid caves, once more a mermaid yourselves, the Cutie Knights quickly find themselves in battle against a group of mermaids that have seen through their disguise. Following the battle you are free to begin exploration of the new area. This time you are starting in the outskirts of mermaid territory, one map off of where you were last time in the original visit. Aside from the path to the old area, this map has entrances leading to five other caves. Split into a north and south half, the south half has a cave on each side. The north half has cave entrances in the northwest, direct north, and northeast, with the path leading to the old area on its eastern end.
The group's goal is to eventually make their way to the throne room and confront the Mermaid Princess. Between the starting point and the throne room, there are two checkpoints that need to be passed, with each having several valid options. It is also possible to head there directly and pass the checkpoints with brute force instead, but doing things the proper way is significantly easier, and there is also a side quest in the area which unlocks a possible neutral ending.
Of the caves with items relevant to the main quest, you will want to visit the southwest cave and the north cave to get the Healer's Kit and the Old Shield respectively, along with other odds and ends in each cave. The northeast cave has a weapon for Alice, the Sacrificial Dagger. The northwest cave will start a side quest involving additional story for the area that also leads to an optional ending. The southeast cave only has some lumes. Check the treasure section of this page below for a summary of what is in each cave.
Heading into the northwest cave the Cutie Knights walk into a troop of mermaid guards attempting to apprehend a dissenter, Eliza. This quickly turns into getting involved in the fight, and in the aftermath Veronica's distaste for the Mermaid Princess convinces Eliza that you are all on the same side. Agreeing to help her will have her wanting you to deliver Eliza's Parcel to a traitorous guard in the village. You do not have to accept right away and can talk to her again to accept it if you turned her down initially.
When you are finished checking out the caves, head to the east end of the northern half of the cave and head down to the old area. There is nothing new or interesting in this area the second time around. Attempting to head through any of the old passageways will just have Veronica warn you off from getting lost. So just treat the map as a hallway and head immediately down to the southeast exit to head to the village.
Mermaid Village
The mermaid village presents the first of the two checkpoints you must pass. At the south end is the entrance to the noble district where the throne room is located, but the path is being watched over by a pair of guards. You will have to either find an excuse to make them let you pass, or try and brute force your way through. Trying to force your way through involves a series of eight consecutive fights, some of which include truly ridiculous amounts of enemies. While this is not technically impossible, and will skip you directly to the throne room, it will be a tedious, grueling, and extremely difficult gauntlet.
If you choose not to force your way through, then there are two things that can be done in this area to secure your passage in, and there is also the continuation to the rebellion questline if you chose to deliver Eliza's package:
- Merchant Pass: The northwest dwelling in the village contains a shop, the Sunken Treasure. Speaking with the mermaid on the right side will reveal that she is the terminally ill younger sister of the Shopkeeper Mermaid. If you picked up the Healer's Kit earlier, then when speaking with the shopkeeper you can choose to show it to her, using it up to cure her sister. Speaking with her again after that and telling her of your need to access the Noble District will have her giving you a merchant's pass that will allow you to bypass the guards. Do note that the Healer's Kit if not used here can also be used to pass the guard in front of the throne room later, but there are other options available for that as well. Check the Noble District section below to see what they are.
- Enchantress: The northeast entrance is a temple holding examinations for enchantress hopefuls. If you were blocked by the guards down south, the Cutie Knights will consider entering. Don't worry about the warning at the entrance and simply head in. For the examinations, each Cutie Knight will take turns singing to assess their potential. If Eric(a) has the Feminine Voice Transformation or Lyrical Choker then their beautiful voice will be noted, but either way Eric(a) lacks any talent in singing at all. Natalie and Veronica both have good voices and singing talent, but lack the spark for song magic. Alice however successfully meets their criteria. They will then ask Alice to test her abilities on Eric(a) as the least talented one. Agreeing will net you a purple pearl that can allow you past the guards, and leave some lingering magic on Eric(a) which can combine with the later parts of the rebellion quest line to lead to an additional Feminine Mannerisms Transformation.
- Rebellion: Eliza asked for the package to be delivered to a guard that answers a particular code phrase correctly. Do make sure to give it to the correct guard, as it is possible to give it to the wrong guard and no longer be able to progress the quest. If on the quest, Eric(a) will automatically prompt each guard with the code phrase "The currents are cold today." The correct return password you are looking for is "But I have a feeling they will get hotter soon." The correct guard should be the one just north of the centre monument, but pay attention to the phrases to be sure.
Noble District
Now in the Noble District, you are only a stone's toss away from the end. The centre structure contains the throne room, with its entrance on the south side and blocked by a guard. For other points of interest, there is the jail in the northeast that you can enter (but lacks anything worthwhile inside), the enchantress HQ in the southeast you cannot approach out of wariness, and if you continued the rebellion questline then Eliza is in the southwest corner. Similarly to before, it is possible to directly confront the guard and force your way in, this time with only four fights instead of eight, since you did half the necessary puzzles and are much closer now. The same considerations apply.
If you choose to do things the proper way, then your options are:
- Healer's Kit or Old Shield: Both these items bare the symbol of the old queen, items that the Mermaid Princess highly covets. For the former it does require not having used the Healer's Kit to heal the sick mermaid. Hand over either of these items to the guard to be let into the throne room.
- Rebellion: Head southwest to meet up with Eliza. If you agree to help her, then Eric(a) is to act as her "wife" as the two head over towards the enchantress HQ. This results in more magical shenanigans being applied to Eric(a)'s mind, which if Alice did end up using Eric(a) as a training dummy during the enchantress exams will end up having a Feminine Mannerisms Transformation applied later. Eliza ends up taking the opportunity to smash several of their spell orbs before running off with Eric(a) in tow. In the after action scene Eliza ends up deciding she wants some "action" of a different kind and jumps Eric(a). Normally this stops in short order with the rebellion leader Salenia showing up soon after, but if Eric(a) is in the Succubus Shift you can get some alternate dialogue and choices, although nothing of consequence changes. In the end, Salenia will provide a conch signifying an invitation to a royal audience to allow you past the guard.
Throne Room and Aftermath
Once in the throne room, things will quickly devolve into a fight with the Mermaid Princess. Assuming you win then Salenia will appear after the battle to complete the coup, presenting the royal regalia that mark the true queen, deposing the current Princess and having her dragged off to the dungeons. Under normal circumstances Salenia will then assume the throne herself. However, if you have completed all previous parts of the rebellion questline then she will instead offer the crown to Eric(a), leading to the Mermaid Queen Ending if accepted. With or without Eric(a) the Cutie Knights will then return to camp, bringing the poor mind controlled Magical Girl now known only as "Pussylicker" back with them.
Back in camp, you can speak with Marianne again in the morning to receive an update on how things have turned out with Pussylicker, as well as to receive a reward for your good work. In Marianne's Junk Room you can speak with her while she searches to learn about some of the things she has stashed away in this room, or just wait for her to finish. The available rewards of which you can choose one are: Breast Reduction Elixir, Gold Digger's Panties, Mind Cleansing Pearl, and Moonsilver Wand. Note that the Breast Reduction Elixir does require at least one point in the Breast Growth Transformation to be given as an option, as well as less than fifteen Acceptance.
Enemies
Since the entire dungeon is underwater, all combatants in it are immune to 
Wind skills.
- Nereid
- Losing to the Nereid results in the Beach Babes Game Over.
- Mermaid Enchantress
- Mermaid Warrior
- Mermaid Princess
- Boss enemy. Not a Magico.
- Winning provides the Mermaid Princess Scale.
Treasure
Outskirts Main Area
- 600 lumes - Chest in the southwest corner.
- Fairy's Dust - Chest in the north path.
- Purple Crystal x2 - Chest in the northwest corner.
- Red Seaweed Extract - Chest to the east of the starting point, near the south edge of the map.
- White Crystal x2 - Chest just west of the passage in the centre of the map.
Southwest Cave
- 1200 lumes - Chest in the northwest corner.
- Healer's Kit - Chest in the open west of the entrance.
Southeast Cave
- 2000 lumes - The only chest in the small map.
Northwest Cave
- Eliza's Parcel - Agree to help Eliza.
North Cave
- Old Shield - Chest in the northwest corner at the end of the long hazardous path.
- Seashell Bra - Chest in the chamber to the west of the entrance.
Northeast Cave
- Haunted Skull - Chest in the northeast area.
- Purple Crystal - Right chest in the northwest area.
- Sacrificial Dagger - Hidden in the green corals in the northeast area. Each path in this room has a pair of red+green chests with similar content; this item is located where the green chest should be on this path.
- Seal of Focus - Left chest in the north area.
- Seal of Purity - Right chest in the north area.
- White Crystal - Left chest in the northwest area.