Before you start the next level, you get a little story:

Uncover the lore of Doom 64. This section provides the story that plays before you begin each new level, offering context to your demonic battles.

Before you start the next level, you get a little story:

The demonic invasion of Earth has been halted, but the Mother of All Demons has opened a portal to Hell on the moons of Jupiter. The UAC has sent the last surviving Marine to investigate and destroy the source of the invasion. You are that Marine.