With the conclusion of "Teen Titans" in 2006, the fourth season introduces Dick Grayson (voiced by Evan Sabara) The opening episode of the season focuses on Dick Grayson's origins as Robin. Through the loss of Grayson's parents in a murder made to look like a circus accident, he is adopted by Bruce Wayne, who assists Grayson in bringing down the mafia boss responsible for his parents' deaths. The second episode in the season leads to Batgirl officially becoming part of the team, with each member divulging his or her secret identity to the others. This reveal eventually leads to Batgirl and Robin forming a sibling-like rivalry. One of the highlights of season 4 is the episode "Artifacts", which described a possible future in the year 3027. Featuring flashback scenes to 2027, the audience gets a glimpse of an older Batman, Dick Grayson operating as Nightwing (voiced by Jerry O'Connell), and Barbara Gordon, now bound to a wheelchair and having become the Oracle (voiced by Kellie Martin). The episode also features elements from Frank Miller's "The Dark Knight Returns", including the designs of both the Batman costume and the Batmobile. In the season finale, an alien invasion by entities called "The Joining" causes Batman to work alongside another superhero from the DC Universe and an original member of the Justice League, Martian Manhunter, to thwart the aliens' plans. This is when Batman becomes part of the Justice League of America.