Another Century's Episode 2 (アナザーセンチュリーズエピソード 2 Anazā Senchurīzu Episōdo Tsu), abbreviated as A.C.E.2, is a third-person mecha action video game produced by Banpresto and developed by From Software. It is the sequel to the popular 2005 game Another Century's Episode.
A.C.E.2 is not a direct sequel to its predecessor, as it involves its own original plot, as well as covering events that already happened in A.C.E., albeit differently. This is not an unusual occurrence, as Banpresto has done this with most entries in the Super Robot Wars franchise; most of the games in that series are not connected by an established continuity (exceptions include the Super Robot Wars Alpha games and the titles in the Super Robot Wars Classic Timeline), with remakes or updates abounding.
However, there are some links back to A.C.E. in the sequel. The Ark series, including the main character's Gunark, use as their power source the super-volatile substance E2, which was introduced in the first game. Additionally, most of the A.C.E. original enemies return, this time with a backstory, while in A.C.E. they were simply there to add to the challenge.
Source: Wikipedia, "Another Century's Episode 2", available under the CC-BY-SA License.