There're several timelines, interesting
1969 - the kids invent the symbol of true friendship
1973 - some girl is kidnapped, "20th century boy" is playing
1997 - Kenji finds the symbol on the property of a missing family
???? - some girl in pants witnesses a monster
21st century - UN meeting, the ones who saved humanity are introduced (I assume they are the protagonists in adulthood)
Probably 21st century again - the mysterious "friend" is floating on what looks like a cult meeting