Steins;Gate: Heni Kuukan no Octet

Around a month after the time machine disappearance in Steins;Gate worldline, Okabe is struck with the familiar vertigo, Reading Steiner, and after the vertigo disappears, he gets a D-Mail from 2025 Okabe Rintarou that explains this worldline is in dangerous future which is worse than Dystopia or World War III. His mission is to retrieve an IBN 5100 from a person known as Neidthardt.

(Source: Steins;Gate Wiki)