A general summary of the plot, based on my limited Kancolle lore knowledge, as practice for my elementary Japanese. I initially got my own copy for the comedy, but it's surprisingly compelling given its theme on family and Naka character development, who has mainly served as the resident comic relief/recurring harmless antagonist in previous works. I welcome any corrections and hope this encourages a translation of the other Betemius works.
Fubuki wakes up from her nightmare in which Sendai defected to the Abyssals, which include her younger sister ships Jintsuu and Naka. She tells Mutsuki and Yuudachi about her dream, only to notice Sendai being shipnapped by her long-lost sisters. On her side of things, Sendai is vehemently expressing her opposition to joining her sisters, but they insist on going back to their base with their sister with a little physical coercion. The destroyers arrive (with Yuudachi still brushing away) and begin their counterattack, which backfires when their salvos straddle Sendai's face. Sendai's sisters take this stroke of luck in stride and escape with their prize. The destroyers panic and Fubuki starts to call for reinforcements, but Mutsuki reminds her that it is their fault that the situation deteriorated to this extent and warned that they would get court martialed for friendly fire and general incompetence. Instead, she proposes her own bold plan to rescue Sendai from the Abyssal base, which requires them to find a guide.
Down in the Abyssal base, Sendai wakes up and is greeted by Naka and Jintsuu, who proudly welcome her into the Abyssal fleet's company. They play with her emotions a bit by joking about how she has become one of them, but after some reassurances mainly aim to convince her to join them officially by mentioning how the Abyssal Fleet is a fun company with familiar friendly faces like Harusame. Said Abyssal friend was idling around the Naval Base perimeter wondering how her sisters were faring when Mutsuki suddenly emerges from underwater and begins interrogating her on Sendai and the Abyssal Base's location. Yuudachi and Fubuki arrive to Harusame's relief (especially since she and Poi are both Shiratsuyu-class destroyers), but Mutsuki doesn't let up.
Meanwhile, Jintsuu continues her attempts to win over Sendai by addressing superficial concerns like food (home-made Abyssal kombu) and sleeping conditions, but as the negotiations continue to stall, Naka loses her patience and points out that their decision to kidnap her stemmed from Harusame reporting that Sendai was spending most of her time facing the ocean, while they themselves really missed their older sister. However, Sendai admits that while she saw that her sisters enjoying life as Abyssal ships, she couldn't give in because they were fundamentally enemies. Naka doesn't take this rejection well, and reaches the conclusion that if that's the main thing standing between their family reunion, then sinking Sendai would solve it outright. At that moment, the destroyers come barreling in, and Naka's response is interrupted by a Harusame to the face. While Yuudachi tries to console her battered and abused Abyssal sister, Sendai grabs the other destroyers along with the distraught Poi and apologetically blows past Jintsuu. Naka catches up to them, but she holds her fire at the last moment, allowing the escapees to make their way back to the surface as an act of sisterly love.
Yuudachi's regret over being separated from Harusame again causes Sendai's grief to swell back up, but she keeps a strong face while they return to base. After the successful rescue operation, the destroyers stuff their faces full as Sendai does the explaining, while an unexpected and well-made yet familiar-looking food package containing kombu arrives for the last Sendai-class sister.
In the omake, Fubuki witnesses Poi turning down a dejected Harusame who had presumably offered to become Abyssals together.