HeroSite source Mr.Video has revealed that the titles for episodes 20 and 21 are:

• Chapter Twenty “String Theory”
• Chapter Twenty-One “The Hard Part”

Also the story I’m hearing is that both episode 20 and 21 take place in the future. This makes sense with the pictures of Isaac about to become a new addition to Sylar’s brain collection in episode 19 (as seen in the NBC spoiler pictures) and the spoilers that Kristin released stating that the next major Hero death is in episode 22. This seems to indicate that Isaac’s death will be a cliffhanger at the end of episode 19.

Source: HeroSite.net