You've surely heard by now that Gearbox Software's Aliens: Colonial Marines delivers a story that franchise rights-holder 20th Century Fox regards as official canon. The setup for that story is laid bare in a newly released trailer which is light on gameplay and heavy on exposition.
Colonial Marines is effectively a direct sequel to the events of Aliens, picking up not long after the events of Alien 3. A search and rescue force sent to investigate the U.S.S. Sulaco steps into the aftermath of a Xenomorph bloodbath, both aboard the ship and on the surface of LV-426. The new trailer features lots of nods for franchise fans, so give it a look.