While on the Justice League's orbiting Watchtower, an alert of dual emergencies in Gotham City calls Batgirl and Supergirl into action. They step into the teleporter. In the same moment, something attacks and damages the Watchtower's tech. The two girls return to Earth... but as one combined Super Hero! As they try to undo the mashup mishap, the merged pair also need to stop Two-Face's double-trouble crime spree that's wreaking havoc on the city. Can the two friends work through the fusion confusion and fight as one?