Perhaps around the midway point of the film, ARGUS detects a seismic anomaly, so they send a team to check it out (including Steve Trevor).
Later, Superman hears this seismic activity and tracks the sound to the island. He finds Trevor, the lone survivor, being held captive by the natives of the island.
Superman warns about the sound coming from beneath the island, and the leader, Hippolyta, fears that it may be Doomsday, a 'demon' or 'fallen God' that was buried long ago by the other Gods by using its only weakness against him -- green crystal (and if they hint that Gods/Amazons are the result of Kryptonian genetic manipulation, then perhaps Doomsday can simply be a failed result of this). Now, for some reason, it has awoke and is making its way to the surface.
When Doomsday emerges, it heads for the mainland, and Superman tries to stop it over the ocean, over mountain ranges, etc. Diana is the only one who volunteers to take Trevor back to his home and warn his people.
Meanwhile, Batman gathers the massive liquid kryptonite reserves he has been engineering. After an epic fight, Superman resorts to tackling Doomsday into the pool of kryptonite, though at the expense of his own life.