"Call to Arms" isn't really a mission. But if you click "J" you can see all the "episodes" that are available to you. And here are links to the summaries that each season brought to the game...

Season 5 | Season 4 | Season 3 | Season 2 | Season 1

Probably they came up with "Call to Arms" to have a dual meaning: (1) The launch of free-to-play, thus the call to all new captains to join us; and (2) The confrontation with the Borg via STFs.