Final Fantasy IV -Interlude- is a bonus feature side-game bundled in Final Fantasy IV: The Complete Collection with Final Fantasy IV and Final Fantasy IV: The After Years. It can be played and completed independently of either game from the main menu, and covers events that link the two games.
Interlude uses the same gameplay system as the PlayStation Portable release of Final Fantasy IV, which is based on the Game Boy Advance release. Characters have the same abilities, spell lists, sprites and equipment pool as in the original game, and enemy encounters are similar including dropped items. The game is linear, with airship flight being controlled by NPCs and the player unable to return to most areas after flying to a new location.
Filling the game's bestiary unlocks several entries in the art gallery that is shared between the three games.
Approximately one year after the party defeated Zeromus, Cecil dreams of Rydia entering a Crystal Chamber, where a voice says "Finally, it has a new form." As the voice is about to reveal itself, Cecil's wife Rosa wakes him—they are headed for Damcyan to celebrate the reconstruction of the castle. Meanwhile in the Feymarch, Rydia prepares to leave to attend the ceremony after speaking with the Eidolon Asura.
At the celebration, Cecil meets with Palom, Porom, Yang, an Epopt, the Elder of Mysidia, and Edward's secretary in training. During the celebration a guard arrives to tell Yang that some monks were attacked by monsters on Mt. Hobs. Yang leaves to investigate, Rosa and Cecil accompanying him. They find the monks and heal them, aiding them in fighting the Dad Bomb, and explain they were on their way to Damcyan because in Fabul, Yang's wife Sheila is about to go into labor with their child.
The group returns to Fabul and witness Sheila give birth to Ursula, Yang appointing Cecil as her godfather. They prepare to return to Damcyan when Cid arrives in the Falcon with Palom, Porom, and Luca. Monsters are pouring out of the Sealed Cave, overwhelming the dwarves. Rosa and Cecil prepare to accompany them to investigate, but Rosa falls ill and remains behind to rest. Cecil accompanies the four to the Falcon and they depart to the Underworld. Meanwhile in Eblan, Edge departs the castle.
In the Underworld, Cecil, Cid, Palom, and Porom, enter the Sealed Cave. In the Crystal Chamber they find Rydia collapsed in front of the Dark Crystal. She awakens and has no memory of how she came to the Crystal Chamber, but tells the group she must go to the Tower of Babil. The group leaves the cave, Rydia telling the group that something is calling her, and the Tower of Babil begins to glow ominously. Concerned about Rydia and the tower, the group flies the Falcon there to investigate. Meanwhile, Edge makes his way through the Eblan Cave that connects to the tower, worried about the glow as well and suspecting someone has been to the tower.
At the tower in the Underworld, Cecil and the group climb it, encountering elemental monsters who, when defeated, impart the power of Eidolons to Rydia. As they pass the cannon control room Rydia enters it and three monsters emerge to stop Cecil and the others from getting to Rydia. The three guards merge, and Edge arrives to lend a hand in defeating it. After the battle, they enter the control room to find Rydia is gone. Continuing climbing the tower they find Rydia, who turns out to be an imposter. The woman attacks the party, summoning her Eidolons one by one when they are defeated. When she summons Bahamut, the real Rydia arrives and brings Bahamut to his senses, allowing the group to fight the imposter.
Though she is weakened, the woman vanishes before the party destroys her. As they wonder who she is, a scene shows her in the Crystal Chamber at the top of the tower. She announces to an unknown entity that "the Eidolon system" is complete, before vanishing. Back on earth, in Cecil's bed chamber in Baron Castle, Rosa lies in bed and Cecil asks if she is alright. Cecil goes to the throne room to the other party members and reveals Rosa is pregnant. Everyone is overjoyed and tells Cecil he should start coming up with names. Meanwhile, on Mt. Ordeals, Kain is training in solitude when he hears a voice call out to him.