The movie starts off after the events of Episode 3, when Kisaragi had sunk. The Mikawa Fleet (Furutaka, Aoba, Kako, Kinugasa and Tenryuu) are in the middle of Night Battle. Choukai uses Searchlight and gets medium damage, but they win in the end. A New Fleet Girl gets "dropped", emerging from the sea. It is Kisaragi. Kisaragi is brought back to the temporary base in Solomon Islands, but she is suffering from PTSD and has amnesia, unable to remember anyone from back in the anime, excluding Mutsuki...
(Source: Wikipedia)+ More
Characters & Voice Actors

Akagi
Main
Fujita, Saki
Japanese
Fubuki
Main
Uesaka, Sumire
Japanese
Kaga
Main
Iguchi, Yuka
Japanese
Kisaragi
Main
Hidaka, Rina
Japanese
Mutsuki
Main
Hidaka, Rina
Japanese
Akashi
Supporting
Taneda, Risa
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