As the ferry of his school's field trip collides with something underwater in a dense fog, [[OrdinaryHighSchoolStudent Yamada]] is thrown overboard and loses consciousness. Some time later, he awakes to find himself on a kilometer-wide [[DesertedIsland uninhabited island]]. As days pass without sign of rescue, Yamada takes matters into his own hands and sets about creating a boat as his rations steadily decrease... when he unexpectedly finds a girl from his class, Imaise Chika, swiping some. For the next few days they cooperate in their efforts to survive and escape, until something [[TimeTravel very unexpected happens]].

Later, they are joined by Shibata, an apathetic teacher at their school; Nori, a classmate highly infatuated with Yamada; and Imaise Miki, Chika's older sister and Nori's [[JapaneseHonorifics senpai]] in the school's Track & Field Club.

Written and drawn by Okazaki Takeshi, ''Let's Lagoon'' at first glance appears to be an [[DeusExMachina arbitrarily]] [[InMediasRes set up]] island survival/drama {{manga}}. A few chapters in, however, things take a highly interesting turn.
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!!Includes examples of:
* ACupAngst: Nori's reaction, when she realised that Chika's deep cleave wasn't even supported by a bra.
* AccidentalPervert: When Yamada and Chika [[CrashIntoHello meet]].
* BlindWithOutEm: Chika. A rare played for drama example. Chika lost her glasses when she fell off the ship. The main reason Yamada didn't confess to Chika while they were alone, was because he thought she'd have no choice but accept, because she wouldn't have been able to survive on her own due to her poor eyesight.
* CassandraTruth: [[spoiler: Nobody believes Yamda's talk about a mysterious island when he gets back to civiliation naturally so he has to go back him self to rescue Chika and Shibata.]]
* ChronicHeroSyndrome: Yamada shows hints at this.
* CrashIntoHello: Well, technically, the collision happened shortly after they met.
* DesertedIsland
* {{Engrish}}: The title. Ow. [[note]]The fault is not with the manga itself here; like most loanwords, "let's" is used more broadly in Japanese slang than in English. It just expresses enthusiasm or intent, so what follows doesn't have to be a verb. Knowing this, however, does not make anglophones' ears hurt any less.[[/note]]
* InMediasRes
* {{Jerkass}}: Shibata
* LoveTriangle: Nori->Yamada->Chika
* [[spoiler: NeverTheSelvesShallMeet: When Yamada comes back to the island, he arrives at a point in time where his past self as still inhabiting the island, he resolves to avoid this.]]
** [[spoiler: Averted. Turns out that the two Yamadas did/will indeed met each other. They met when back in chapter 5 when(Past)Yamada took a nasty tumble. (Future)Yamada took the opportunity to give... er, himself... some advice, knowing that his past self would think that whole encounter was a pain-induced hallucination.]]
* [[spoiler: TimeTravel: It seems the fog surrounding the island can cause this.]]
** [[spoiler: StableTimeLoop]]
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