Protagonist is someone who has a golden finger like maybe a system or a special item or special book or whatever(I suggest not to use the usual golden finger of having immense talent). while the usual story happened, the protagonist's friend or maybe even lover have a change in personality.
so in the far future maybe the protagonist died by someway or maybe he turned evil or pyschotic after killing the boss.
if its the former then the friend had to somehow save him by maybe revealing a secret for lets say a trick to use the protagonist golden finger more efficiently or secret technique which was only found in the far future.
or if its the later than the friend had to somehow killed the protagonist as he now had the advantage of knowing his golden finger more than the protagonist.
Its really up to you how u want to proceed w the story really