Michael Dorn reprises his role as Worf, as a collaboration between the Klingons, Romulans and Federation open up a found Dyson sphere, who then activates and warps itself to the edge of the Delta Quadrant.
It is manned by Romulans who discovered it afterwards, and the location is learned of a second Dyson sphere, the Jenolan one (named after the TNG episode where Mr Scott appeared in).
This sphere however is first attacked by the Undine, but they are repelled and an alliance is formed to man this sphere, and to pose resistance to the Undine thread.
During this conference, the Undine use the Iconian Gateway to attack Earth, and the allies rush in to defend Sol system.
This turns out to be a diversion, as their true target was the Klingon homeworld of Qo'Nos, as they uncovered the Undine plot years ago. The alliance moves in and saves the planet from a new Undine planetkiller weapon.
However, during the ceremony of victory an Iconian appears, warning those present that they will be exterminated if they continue their actions. This results in the end of the war between the Federation and the Klingon Empire, and the beginning of the war against the Iconians.