Dr. Daisuke Serizawa (Akihiko Hirata)’s Timeline and Summary

Serizawa sees off Emiko, Ogata, and Yamane as they head for Odo Island.
He then spends most of the story researching oxygen and terrorizing fish in his laboratory—at least, that's our best guess for what he's up to.
Hagiwara pops in to interview Serizawa about his research, but Serizawa says there's nothing much to talk about. He's just sciencing, yo. No biggie. (That's a rough translation, by the way.)
When Haigwara leaves, Serizawa shows Emiko what he's really up to. Whatever happens in that lab freaks her out, but she promises not to tell anyone.
She lied. After witnesses the devastation of Godzilla's attack on Tokyo, she tells Ogata about the Oxygen Destroyer.
In order to kill Godzilla, Ogata asks Serizawa for the Oxygen Destroyer.
Serizawa refuses at first, but after seeing a news report on the death and destruction, he reluctantly agrees.
Burning his notes, he proclaims that the Oxygen Destroyer will never be duplicated.
Serizawa and Ogata scuba down to the ocean floor and find Godzilla chilling out.
Serizawa signals for Ogata to surface and activates the Oxygen Destroyer.
Cutting his own line, Serizawa dies with Godzilla to ensure that his knowledge will never be used to create another device of mass destruction.
Before he dies, he wishes Ogata and Emiko happiness together.