As Lillie narrates the events of the previous episode, the group exits the Ultra Wormhole and enters Ultra Deep Sea, a dark location filled with floating islands of jagged rocks. Just as the group lands, they find themselves surrounded by several Nihilego. Rotom Pokédex warns of the danger, and Gladion clarifies that the Ultra Beasts are UB01: Symbiont – the same ones responsible for his mother's kidnapping. Meanwhile, Professors Kukui and Burnet, still at the Altar of the Sunne, have set up a temporary Ultra Wormhole monitoring station. One of the machines fires a location marker to aid Ash and his friends upon their return. Team Rocket watches the professors from their cliff-side view. As they try to retreat, with Ash now missing, Bewear hugs them tightly to immobilize them.
Back at Ultra Deep Sea, Gladion demands the group split up, though the idea concerns Lillie. Sophocles states an investigation is in order, before one of Nihilego's tentacles brushes against him. Ash's confidence only irritates Sophocles, though Ash explains that the group have their Pokémon. This spurs the group into calling out all of their Pokémon. Nebby roars, just as Lusamine appears and her eyes open, though she is trapped and bonded with a Nihilego. Lillie calls out and Rotom confirms Lusamine's identity with a facial recognition feature.
Solgaleo roars once more; this time, Lusamine demands the group stay away from her. Lillie runs towards her mother, and Ash and Gladion attempt to stop her. As a defense, Lusamine blocks the path with jagged rocks, but Ash manages to save Lillie from injury. Gladion's Umbreon uses Dark Pulse, which shatters the stones. Gladion theorizes that Nihilego may be draining his mother's energy, though he clarifies little is known of Ultra Beasts in general. Ash moves forward, though Kiawe notes that the group may be provoking Lusamine. She continues to retreat, exclaiming that she finally got to meet an Ultra Beast and will not give it up, leaving a rocky staircase for the group to follow. Mallow calls Lusamine's behavior childish; Gladion adds that Lusamine may consider the Ultra Beast a toy that she won't part with, and consequently considers the group as a threat.
The group finally catches up to Lusamine, and Lillie commands Snowy to use Powder Snow. Lusamine defends herself with her rock shield and then shouts out that she hates everyone. Lillie is initially taken aback by this, but Mallow offers support and Lillie concludes Lusamine must be illogical. Pikachu's Thunderbolt, Turtonator's Flamethrower, Marowak's Bonemerang, and Gladion's Lycanroc's Stone Edge all prove to be useless as well. Nebby then steps forward, glows, and jumps through the barrier, much to everyone's amazement. Lusamine summons her Salazzle, which is surrounded in an aura. Turtonator counters Salazzle's poison attack with a Flamethrower, which allows Lusamine to flee again. Salazzle uses another attack and evades Turtonator's Dragon Tail, though it is then hit by Marowak's Flare Blitz. Kiawe volunteers to stay behind to take care of Salazzle while the rest of the group continue on.
The group catches up to Lusamine once again, and she calls upon her Milotic, Mismagius, and Lilligant. A Thunderbolt and Hydro Pump are launched, though the group evades the attacks. Lana has Popplio use Bubble Beam, while Mallow calls for Magical Leaf, though Liligant's Petal Dance blocks the attack. Mismagius uses another Thunderbolt, though it is absorbed by Togedemaru. Ash, Lillie, and Gladion continue following Lusamine, leaving the others and Rotom to confront Lusamine's diversions. Popplio's Bubble Beam hits Liligant, and Steenee's Double Slap combats Milotic's Aqua Tail. Just as Mismagius laughs at the group's induced confusion because of Liligant's Teeter Dance, Ash's Litten and Rowlet join the match. Liligant's Sweet Scent instantly attracts Rowlet, much to everyone's frustration. Meanwhile, Kiawe's match against Salazzle ends after a combination between Turtonator's Inferno Overdrive alongside Marowak's Bonemerang.
Meanwhile, Gladion, Lillie, and Ash re-encounter Lusamine, and she sends out her Absol. Gladion remarks that it was a sparring partner to his Umbreon. Absol makes the first move, which collides Umbreon's Dark Pulse. Gladion then sends Nebby, Ash, and Lillie on their way. With the battle still raging, Absol fires an Ice Beam, which freezes Silvally on the spot, followed by Mean Look, which is used on Umbreon and Lycanroc.
Lusamine is again confronted by Ash and Lillie; she continues to express her frustration towards them, and calls upon Clefable. Lillie tells Ash that she will battle against Clefable, and refers to it as her best friend. After Lycanroc intercepts Clefable's Moonblast, Ash leaves it with Lillie and rides on Nebby towards Lusamine. Clefable attacks with Dazzling Gleam, though Lycanroc again blocks the attack. Both Snowy's Powder Snow and Lycanroc's Rock Throw hit their target, pushing Clefable back. Lillie grabs a Poké Doll from her bag, then calmly and defiantly approaches Clefable. She expresses her hope that Clefable will stop fighting when it reminisces about their past together. However, Clefable begins to launch another Dazzling Gleam. Lillie gestures to Lycanroc and Snowy to stay back as she embraces Clefable and is engulfed in the attack's light. Her persistence pays off, and Clefable returns to its usual state and smiles. Meanwhile, Ash and Pikachu continue to pursue Lusamine aboard Nebby.
Outside the Ultra Wormhole, Burnet gets Kukui to send out Braviary. Burnet heals Braviary with a Potion, places a band-aid on its talon, and thanks its efforts against Hakamo-o.