Emily is correct.
A little background if you have not seen it in a long time:
Luka went to Africa with a doctors without borders type program. He convinced Carter to come to Africa too. Abby was not happy about this. Last year's finale/this season opener found Carter coming back after a 2 (?) week tour in Africa and leaving Luka in the war ravaged Congo. When he returns he gets a call that Luka has been murdered. Carter immediately decides to return to Africa, which yet again pisses Abby off, to retrieve Luka's body. When he gets there he has a hard time finding where Luka's body is. He finally manages to track him down, and there had been a mistake. Luka was alive (the rebels thought he was a priest because he was wearing a cross the mother of a patient had given him and did not kill him) but he was very sick with Malaria. Carter managed to get Luka, and the woman he was seeing in Africa (you probably saw her in tonight's episode when Abby came over saying she was invited for dinner) out of Africa and back to Chicago. Carter had some revelation though and decides that "for now" his time is better spent in Africa where he can really make a difference. He sent home a break-up letter to Abby.
Other notable happenings: Dr. Corday is "seeing" (read:sleeping with) Dr. Dorset (what a hottie!). Dr. Romano is still a jerk, but he had his arm chopped off after they were unable to fully repair it from last year's helicopter accident and is having issues getting the prosthesis he wants. There are about 1000 new med students this season. Dr. Pratt was flirting with some of them, mainly Neela and this bothered Dr. Chen. They have now broken up. Weaver and Lewis have not had much going on in the last few episodes.